Core - Cef.WaitForBrowsersToClose increase Wait Timeout

- After upgrading to M102 shutdown appears to be taking longer, so we'll increase
  the Wait timeout from 500ms to 750ms to allow CEF a little more time to finish processing.

Author: amaitland

CefSharp.Core.Runtime/Cef.h

@@ -863,7 +863,7 @@ namespace CefSharp
                 _disposables->Clear();
 
                 //Wait for the browsers to close
-                BrowserRefCounter::Instance->WaitForBrowsersToClose(500);
+                BrowserRefCounter::Instance->WaitForBrowsersToClose(750);
 
                 //A few extra ms to allow for CEF to finish 
                 Thread::Sleep(50);

CefSharp.Test/OffScreen/OffScreenBrowserBasicFacts.cs

@@ -149,6 +149,8 @@ public void BrowserRefCountDecrementedOnDispose()
 
             Cef.WaitForBrowsersToClose();
 
+            output.WriteLine("Browser Count {0}", BrowserRefCounter.Instance.Count);
+
             Assert.Equal(0, BrowserRefCounter.Instance.Count);
         }