The "children transform layer" needs an anchor point

This layer houses what used to be the sublayer transform. It needs to be applied
about the layer's anchor point, so we must set it.

BUG=350815

Review URL: https://codereview.chromium.org/193663004

git-svn-id: svn://svn.chromium.org/blink/trunk@168943 bbb929c8-8fbe-4397-9dbb-9b2b20218538