base

Fix up the WebCoreThreadWatchdog

Make the WebCoreWatchdog aware of the WebViews it is monitoring
(rather than the Activity context which may become stale) and
ensure that the code for the prompt dialog is run on the UI
thread.

Bug: 6420310
Change-Id: Ied003938edb04858c85bcc2491c4b2c4c0ede6eb