Remove obsolete WorkspaceEventHandler::destroyed_

BUG=None
TEST=None

Review URL: https://codereview.chromium.org/103943010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240779 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240779