WinForms - Allow creation of custom ChromiumHostControl instance

- Add new CreatePopupChromiumHostControl delegate which can be passed to
  LifeSpanHandler.Create to create a new ChromiumHostControl instance.
  You can create a class that inherits from ChromiumHostControl and add
  additional functionality.

LifeSpanHandler
  .Create(() =>
  {
     return new ChromiumHostControl
     {
       Dock = DockStyle.Fill
     };
})

Author: amaitland

CefSharp.WinForms/Handler/LifeSpanHandler.cs

@@ -37,6 +37,13 @@ namespace CefSharp.WinForms.Handler
     /// <param name="browser">browser</param>
     public delegate void OnPopupDestroyedDelegate(ChromiumHostControl control, IBrowser browser);
 
+    /// <summary>
+    /// Called to create a new instance of <see cref="ChromiumHostControl"/>. Allows creation of a derived
+    /// implementation of <see cref="ChromiumHostControl"/>.
+    /// </summary>
+    /// <returns>A custom instance of <see cref="ChromiumHostControl"/>.</returns>
+    public delegate ChromiumHostControl CreatePopupChromiumHostControl();
+
     /// <summary>
     /// A WinForms Specific <see cref="ILifeSpanHandler"/> implementation that simplifies
     /// the process of hosting a Popup as a Control/Tab.
@@ -49,7 +56,13 @@ public class LifeSpanHandler : CefSharp.Handler.LifeSpanHandler
         private OnPopupDestroyedDelegate onPopupDestroyed;
         private OnPopupBrowserCreatedDelegate onPopupBrowserCreated;
         private OnPopupCreatedDelegate onPopupCreated;
-        
+        private CreatePopupChromiumHostControl chromiumHostControlCreatedDelegate;
+
+        public LifeSpanHandler(CreatePopupChromiumHostControl chromiumHostControlCreatedDelegate)
+        {
+            this.chromiumHostControlCreatedDelegate = chromiumHostControlCreatedDelegate;
+        }
+
         /// <inheritdoc/>
         protected override bool DoClose(IWebBrowser chromiumWebBrowser, IBrowser browser)
         {
@@ -168,10 +181,15 @@ protected override bool OnBeforePopup(IWebBrowser chromiumWebBrowser, IBrowser b
             //We need to execute sync here so IWindowInfo.SetAsChild is called before we return false;
             webBrowser.InvokeSyncOnUiThreadIfRequired(new Action(() =>
             {
-                var control = new ChromiumHostControl
+                ChromiumHostControl control = chromiumHostControlCreatedDelegate?.Invoke();
+
+                if (control == null)
                 {
-                    Dock = DockStyle.Fill
-                };
+                    control = new ChromiumHostControl
+                    {
+                        Dock = DockStyle.Fill
+                    };
+                }
                 control.CreateControl();
 
                 onPopupCreated?.Invoke(control, targetUrl);
@@ -238,10 +256,14 @@ public LifeSpanHandler OnPopupDestroyed(OnPopupDestroyedDelegate onPopupDestroye
         /// of implementing directly you will need to inherit from <see cref="CefSharp.WinForms.Handler.LoadHandler"/>.
         /// As it provides base functionality required to make <see cref="ChromiumHostControl"/> events work correctly.
         /// </summary>
-        /// <returns>LifeSpanHandlerBuilder</returns>
-        public static LifeSpanHandlerBuilder Create()
+        /// <returns>
+        /// A <see cref="LifeSpanHandlerBuilder"/> which can be used to fluently create an <see cref="ILifeSpanHandler"/>.
+        /// Call <see cref="LifeSpanHandlerBuilder.Build"/> to create the actual instance after you have call
+        /// <see cref="LifeSpanHandlerBuilder.OnPopupCreated(OnPopupCreatedDelegate)"/> etc.
+        /// </returns>
+        public static LifeSpanHandlerBuilder Create(CreatePopupChromiumHostControl chromiumHostControlCreatedDelegate = null)
         {
-            return new LifeSpanHandlerBuilder();
+            return new LifeSpanHandlerBuilder(chromiumHostControlCreatedDelegate);
         }
     }
 }

CefSharp.WinForms/Handler/LifeSpanHandlerBuilder.cs

@@ -9,7 +9,12 @@ namespace CefSharp.WinForms.Handler
     /// </summary>
     public class LifeSpanHandlerBuilder
     {
-        private readonly LifeSpanHandler handler = new LifeSpanHandler();
+        private readonly LifeSpanHandler handler;
+
+        public LifeSpanHandlerBuilder(CreatePopupChromiumHostControl chromiumHostControlCreatedDelegate)
+        {
+            handler = new LifeSpanHandler(chromiumHostControlCreatedDelegate);
+        }
 
         /// <summary>
         /// The <see cref="OnPopupCreatedDelegate"/> will be called when the<see cref="ChromiumHostControl"/> has been