various fixes for browser test harness

* the definitive way to determine page load finish seems to be
  onPageLoadFinished AND onProgressChanged==100, changing test
  harness logic to reflect that

* webview does not seem to be happen if ClearCache is called too
  often (or for certain pages?). It may complain that failed to
  delete blahblah, then the test harness goes crazy. So changing
  cache clear to after an entire iteration of web pages have been
  loaded

* browser will load default home page, which may be disruptive
  to the page load process. So for the test, the browser is now
  started with initial intent to visit about:blank, which does
  not trigger any page load at all

Change-Id: I1cfbd8d0196a05caea6a1c5137f45f1f59cc4c5f
1 file changed