Revert "Separate compositedLayerMapping allocation from computing compositing requirements."

Speculative revert of blink rev 162390 if this may be causing crashes
in renderer process.

TBR=vollick@chromium.org

BUG=322232

Review URL: https://codereview.chromium.org/99173005

git-svn-id: svn://svn.chromium.org/blink/trunk@163039 bbb929c8-8fbe-4397-9dbb-9b2b20218538