Fixes bug in instant that resulted in flickery fade. The problem would

happen if instant wouldn't load the current url and you continued
typing. For example, if the search engine doesn't support the instant
API on each key press you would see the page fade out, then snap in.

The fix is to make sure instant stays active until destroyed. That way
if the preview is no longer valid, but instant is still active the
page can remain faded out.

This also adds a couple more tests for better coverage.

BUG=66720 66539
TEST=see bug

Review URL: http://codereview.chromium.org/5701008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@69201 0039d316-1c4b-4281-b951-d872f2087c98