Decouple JsResult from the WebViewClassic impl

The majority  JsResult and JsPromptResult are simple data-carrying &
callback classes. Extract the WebViewClassic specific parts dealing
with WebCore thread and CallbackProxy and decouple via abstract interface.

Bug: 6238755
Change-Id: Ibafd18910725f0875e3b59c1b2078173c102cdec
3 files changed