Add a CanHandleAccelerators() method to AcceleratorTarget.

      This eliminates the need to register/unregister accelerators based on visibility state changes from views.

BUG=105734
TEST=unit tests

Review URL: http://codereview.chromium.org/8589007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@115837 0039d316-1c4b-4281-b951-d872f2087c98