Uses WeakPtrFactory instead of base::Unretained.

Although TrayIME removes the notification, the callback notification
delegate may survive due to complicated ref-counted-ness of
NotificationDelegate.

Anyways WeakPtr would be better here rather than Unretained.

BUG=336650
R=stevenjb@chromium.org, dewittj@chromium.org
TEST=None

Review URL: https://codereview.chromium.org/138833004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248577 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248577