Instant: Don't allow the page to set suggestions inappropriately.

This guards against internal bug b/8572045. If |last_user_text_| is empty, it
means the user is arrowing up/down the suggestions list (as opposed to typing
text into the omnibox), in which case we reject new suggestions.

BUG=225326
R=samarth@chromium.org
TEST=See bug (the second case in comment #0; and the case in comment #2).

Review URL: https://chromiumcodereview.appspot.com/13813006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@194062 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed