WinForms/WPF/OffScreen - Enable Chromium Site Isolation by default

Previously was disabled by default

Can still be disabled via disable-site-isolation-trials command line argument

Issue #2967

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");