Moves transient window observer methods out of WindowObserver

As transient logic has been moved outside of aura it doesn't make
sense for these methods to be in WindowObserver anymore. Instead they
have been added to TransientWindowManager.

BUG=none
TEST=none
R=ben@chromium.org

Review URL: https://codereview.chromium.org/132013004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244176 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244176