WPF/WinForms/OffScreen - Enable Chromium Site Isolation

CEF has changed it's behaviour slightly to correctly support Chromium Site Isolation
Frame objects (including for the main frame) will now always change after
cross-origin navigation, and the old frame object will be invalidated.
See commit for additional details https://bitbucket.org/chromiumembedded/cef/commits/d9efaee9b9aa

Resolves #2967

Author: amaitland

CefSharp.Core.Runtime/CefSettingsBase.h

@@ -49,11 +49,6 @@ namespace CefSharp
                 _cefCustomSchemes = gcnew List<CefCustomScheme^>();
                 _cefCommandLineArgs = gcnew CommandLineArgDictionary();
 
-                //Disable site isolation trials as this causes problems with frames
-                //being hosted in different render processes.
-                //https://github.com/cefsharp/CefSharp/issues/2967
-                _cefCommandLineArgs->Add("disable-site-isolation-trials");
-
                 //Disable Windows Spellchecker as CEF doesn't support yet
                 //https://bitbucket.org/chromiumembedded/cef/issues/3055/windows-spell-checker-not-working-add
                 _cefCommandLineArgs->Add("disable-features", "CalculateNativeWinOcclusion,WinUseBrowserSpellChecker");