Share more code between boundingBoxForCompositing and physicalBoundingBoxIncludingReflectionAndStackingChildren

This also makes it easier to see what the code is computing.

BUG=

Review URL: https://codereview.chromium.org/356853003

git-svn-id: svn://svn.chromium.org/blink/trunk@177031 bbb929c8-8fbe-4397-9dbb-9b2b20218538