Updating transforms shouldn't rebuild the compositing tree

In Polymer's calculator same app, we mistaken thought we needed to rebuild the
compositing tree even though nothing had changed about our squashing assignments.

This CL speeds up the compositing update for the calculator by 6.5%.

R=chrishtr@chromium.org

Review URL: https://codereview.chromium.org/339203006

git-svn-id: svn://svn.chromium.org/blink/trunk@176787 bbb929c8-8fbe-4397-9dbb-9b2b20218538