Fix webview memory leak when keyboard was shown

Fix webview memory leak when keyboard was shown

Android framework does not seem to release ResultReceiver passed in
InputMethodManager#showSoftInput() after use. As a result, WebView
will not be garbage collected after it's removed, if keyboard shows up
on the screen once.

ResultReceiver cannot be avoided since we want to scroll to the editable
node only after input method window shows up.

This is a fix to weak-reference CVC object (and thus WebView as well) from
ResultReceiver. Unfortunately, ResultReceiver will still be leaked,
but the leak can be significantly reduced.

A test is added to AwContentsGarbageCollectionTest to test that references
to WebView can be removed even when showSoftInput() is called before
garbage collection.

Also, I've manually tested that the new test can catch the memory leak
issue when we strong-reference CVC.

BUG=595613
Review URL: https://codereview.chromium.org/1809013002

Cr-Commit-Position: refs/heads/master@{#381891}
(cherry picked from commit fcc3fbf1652285b2a89eabd092d35d0fdf199ac2)

[Changwan Ryu, 2016-03-18 05:34:59]

Committed patchset #1 (id:1) to pending queue manually as
789e5bfa9b1e9b805a418a47316d00d5d0d5f1ee.