Rename RunCEFMessageLoopOnIdle to DoCEFWork for better understanding.
Running CEFMessageLoop in wxTimer instead of OnIdle.

Running CEFMessageLoop form OnIdle, which may pontentially starve CEF
messageloop when application is busy with other UI-operation.

Change it to timer to avoid the starvation.
Restore OnExit method.

Althrough there is fine to exit the program without calling CEFShutdown on Win/Linux platforms, it's better to
call it explicitly.
Cleanup some commented code.