Make sure we set "should flatten transform" correctly.

The bookkeeping for this property has gotten a little stale, and it's
prevented transforms from propagating in some cases. This CL ensures
that we use the same code everywhere for updating
should-flatten-transform on all applicable layers, and that we call it
at the right times.

BUG=None

Review URL: https://codereview.chromium.org/399733002

git-svn-id: svn://svn.chromium.org/blink/trunk@178400 bbb929c8-8fbe-4397-9dbb-9b2b20218538