Prompt the user to terminate unresponsive pages.

Run a background "watchdog" thread that will check for
the WebCore thread stopping processing messages (this
can happen if a piece of JavaScript goes into an infinite
loop, for example) and offer to close the WebView app when
it detects such a case, much like the normal system ANR.

Bug: 2563868
Change-Id: Ic74813b1e630d657c340a7017a4b0814071eb041