Improve Promise support

[amaitland]

CEF has added support for promises in commit chromiumembedded/cef@fa643b2

• IFrame.EvaluateScriptAsync promise support
• IJavascriptCallback.ExecuteAsync promise support

// For EvaluateScriptAsync 
// Don't use the `return` keyword
Promise.resolve(42);

// For EvaluateScriptAsPromiseAsync you must return a value
return Promise.resolve(42);