[android_webivew] Refactor polling checks in test code.

There really is no good reason to poll for something and not fail
in a test (otherwise we're just adding a sleep). I refactored our
test code to use new AwTestBase methods.

BUG=None
TEST=AndroidWebViewTests
// java-only change, trybots happy
NOTRY=true

Review URL: https://codereview.chromium.org/131743020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245525 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245525