[Android WebView] Limit synthesizing fake page loading events to popups
Only synthesize fake page loading events for popup windows.
This greatly simplifies our logic and eliminates false
positives that can inadvertently crash apps.
Also, add a test for the most commonly used scenario of
doing `loadUrl('javascript:...')` on a uncommitted load,
and rename tests to use shorter names.
BUG=458569,473586
Review URL: https://codereview.chromium.org/
1087513002
Cr-Commit-Position: refs/heads/master@{#325023}