aura: Do not dispatch host-cancel-mode event to the focused window if it's not in the dispatcher's tree.

BUG=347544
R=sky@chromium.org

Review URL: https://codereview.chromium.org/179343006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253992 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253992