[webview] fix testOnJsBeforeUnloadIsCalled flakes

The flakiness appears to have two causes. Firstly, it was possible to
miss the initial page title update because the WebChromeClient was set
after the title change. Secondly, sometimes the synthesised touch events
get turned into a click rather than a touchend.

Fixed so as to:
* Guarantee ordering by setting the WebChromeClient before page load,
  and observing the sequence title changes.
* Trigger on click rather than touchend.

Fixes: 123043623
Test: WebChromeClientTest#testOnJsBeforeUnloadIsCalled
Change-Id: I5d66ebed8b32be54b7267c1e947efc99cb94e04f