[aw] Ensure that WebView APIs do not crash if called after destruction.
The documentation for WebView#destroy() reads "This method should be called
after this WebView has been removed from the view system. No other methods
may be called on this WebView after destroy". However, some apps do not
respect that restriction so this change ensures that we fail gracefully
in that case and do not crash.
BUG=415666
Review URL: https://codereview.chromium.org/
581423002
Cr-Commit-Position: refs/heads/master@{#296029}