Fix composited bounds of reflections.

Previously, they were not taking into account child stacking contexts of the
layer being reflected.

BUG=372837

Review URL: https://codereview.chromium.org/313653004

git-svn-id: svn://svn.chromium.org/blink/trunk@175432 bbb929c8-8fbe-4397-9dbb-9b2b20218538