OffScreen - ChromeRuntime doesn't work with LoadURL

- After switching to the Chrome bootstrap, the browser wouldn't load if you passed in an empty
  string to the constructor, we can defer loading until after another LoadUrl call is made.

Resolves #4832

Author: amaitland

CefSharp.OffScreen/ChromiumWebBrowser.cs

@@ -41,6 +41,17 @@ public partial class ChromiumWebBrowser : IRenderWebBrowser
         /// </summary>
         private bool browserCreated;
 
+        /// <summary>
+        /// When an empty address is passed into the constructor, we defer browser creation
+        /// until we have a Url.
+        /// Issue https://github.com/cefsharp/CefSharp/issues/4832
+        /// </summary>
+        private bool createBrowserOnNextLoadUrlCall;
+        /// <summary>
+        /// BrowserSettings used by createBrowserOnNextLoadCall
+        /// </summary>
+        private IBrowserSettings browserSettings;
+
         /// <summary>
         /// Action which is called immediately before the <see cref="BrowserInitialized"/> event after the
         /// uderlying Chromium Embedded Framework (CEF) browser has been created.
@@ -191,7 +202,10 @@ public ChromiumWebBrowser(HtmlString html, IBrowserSettings browserSettings = nu
         /// that you set <paramref name="automaticallyCreateBrowser"/> to false, subscribe to the event and then call <see cref="CreateBrowser(IWindowInfo, IBrowserSettings)"/>
         /// to ensure you are subscribe to the event before it's fired (Issue https://github.com/cefsharp/CefSharp/issues/3552).
         /// </summary>
-        /// <param name="address">Initial address (url) to load</param>
+        /// <param name="address">
+        /// Initial address (url) to load. If <see cref="string.Empty"/> then cretion of the browser
+        /// will be deferred until <see cref="LoadUrl(string)"/> is called with a non empty Url.
+        /// </param>
         /// <param name="browserSettings">The browser settings to use. If null, the default settings are used.</param>
         /// <param name="requestContext">See <see cref="RequestContext" /> for more details. Defaults to null</param>
         /// <param name="automaticallyCreateBrowser">automatically create the underlying Browser</param>
@@ -230,7 +244,15 @@ public ChromiumWebBrowser(string address = "", IBrowserSettings browserSettings
 
             if (automaticallyCreateBrowser)
             {
-                CreateBrowser(null, browserSettings);
+                if (string.IsNullOrEmpty(address))
+                {
+                    createBrowserOnNextLoadUrlCall = true;
+                    this.browserSettings = browserSettings;
+                }
+                else
+                {
+                    CreateBrowser(null, browserSettings);
+                }
             }
 
             if (useLegacyRenderHandler)
@@ -676,6 +698,16 @@ public void Load(string url)
                 return;
             }
 
+            if (createBrowserOnNextLoadUrlCall)
+            {
+                createBrowserOnNextLoadUrlCall = false;
+                Address = url;
+                CreateBrowser(null, browserSettings);
+                browserSettings = null;
+
+                return;
+            }
+
             //There's a small window here between CreateBrowser
             //and OnAfterBrowserCreated where the Address prop
             //will be updated, though LoadUrl won't be called.

CefSharp.Test/OffScreen/OffScreenBrowserTests.cs

@@ -44,6 +44,23 @@ public async Task ShouldWorkWhenLoadingGoogle()
             }
         }
 
+        [Fact]
+        public async Task ShouldWorkWhenPassingEmptyUrlToConstructor()
+        {
+            using (var browser = new ChromiumWebBrowser(string.Empty, useLegacyRenderHandler: false))
+            {
+                var response = await browser.LoadUrlAsync("www.google.com");
+                var mainFrame = browser.GetMainFrame();
+
+                Assert.True(response.Success);
+                Assert.True(mainFrame.IsValid);
+                Assert.Contains("www.google", mainFrame.Url);
+                Assert.Equal(200, response.HttpStatusCode);
+
+                output.WriteLine("Url {0}", mainFrame.Url);
+            }
+        }
+
         [Theory(Skip = "Not working with Chrome bootstrap")]
         [InlineData("http://httpbin.org/post")]
         public async Task ShouldWorkWhenLoadingRequestWithPostData(string url)