Add autofill preview support for Textarea

BUG=314976
R=isherman@chromium.org

Review URL: https://codereview.chromium.org/112663005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243977 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243977