Fix crash when a layer with a touch handler becomes non-composited.

In r156451 (bug 271747) I added code to clear out hit rects on layers that no longer have handlers.  When a layer with handlers becomes non-composited, it'll still be in our list of layers with handlers, but it will now have a null RenderLayerBacking. Since the RenderLayerBacking was destroyed, there are no rects to clear out, so we just need to skip any such layers.

This CL also removes a related comment which is now obsolete.

BUG=277710

Review URL: https://chromiumcodereview.appspot.com/23072034

git-svn-id: svn://svn.chromium.org/blink/trunk@156602 bbb929c8-8fbe-4397-9dbb-9b2b20218538