Added an assertion verifying that a Canvas2DLayerBridge is not deleted
when still in the Canvas2DLayerManager list.

In order to be able to call Canvas2DLayerManager::isInList,
the method has been changed from private to public.

BUG=385141

Review URL: https://codereview.chromium.org/340093003

git-svn-id: svn://svn.chromium.org/blink/trunk@176442 bbb929c8-8fbe-4397-9dbb-9b2b20218538