Fix idle handlers not being called. The problem was that RenderThread::AddRoute was assuming that each listener was a RenderWidget. That used to be the case originally, but hasn't been like that for a while (i.e. also see ChromeV8ExtensionHandler and ChromePluginPlaceholder).

BUG=333051
R=nasko@chromium.org

Review URL: https://codereview.chromium.org/133483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244193 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244193