cc: Prevent use after free of LayerImpl that had a copy request.

LayerImpls register themselves on the LayerTreeImpl when they have a
copy request, but they were failing to unregister when they were
destroyed before doing the copy. This left them in the list, causing
us to try take the copy requests off a destroyed layer.

Tests:
LayerTreeHostCopyRequestTestDestroyBeforeCopy
R=enne@chromium.org, piman@chromium.org, enne, piman
BUG=309777,326617

Review URL: https://codereview.chromium.org/100163005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240491 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240491