Add some debugging CHECKs to help track down a use-after-free.

Somehow we are left with a LayerImpl* in the
LayersWithCopyOutputRequest() list that has been freed. I have no idea
how, so maybe this will cause some different crash stacks to help
disagnose.

R=enne
BUG=309777

Review URL: https://codereview.chromium.org/131483013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249419 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249419