Use views::corewm::GetToplevelWindow instead of wm::GetActivatableWindow

BUG=none

Review URL: https://codereview.chromium.org/25731006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226698 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226698