Web Animations CSS: Don't animate transforms with box-size dependency on compositor

Running animations of transforms with box-size dependency on the
compositor causes some problems; for example it does not respond
to changing box-size correctly, and we have to layout before the
animation is sent across. This patch forces these transforms run
on the main thread. This means we no longer have to pass the box
size around and makes the code cleaner.

BUG=321840

Review URL: https://codereview.chromium.org/80223002

git-svn-id: svn://svn.chromium.org/blink/trunk@162453 bbb929c8-8fbe-4397-9dbb-9b2b20218538