Need to check whether an extension is loaded in extension service before trying to uninstall it.

      BUG=80868
TEST=ExtensionServiceTest.UninstallingNotLoadedExtension

Review URL: http://codereview.chromium.org/7037037

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@85944 0039d316-1c4b-4281-b951-d872f2087c98