Destroy WorkerDevToolsAgentHost when corresponding worker terminates

The call was missing and now that jam is moving NOTIFICATION_APP_TERMINATING from content/ to chrome/ (http://codereview.chromium.org/10968064/) it became obvious.

BUG=None

Review URL: https://codereview.chromium.org/10989012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@158790 0039d316-1c4b-4281-b951-d872f2087c98