Make 'will-change: contents' suppress compositing in subtree

The contents of 'will-change: contents' elements are expected
to change frequently (e.g. every frame). Consequently, the
creation of composited layers within a 'will-change: contents'
subtree should be avoided where possible. This CL suppresses
compositing for animations and for will-change in descendants
of elements with 'will-change: contents'.

BUG=365885

Review URL: https://codereview.chromium.org/298723011

git-svn-id: svn://svn.chromium.org/blink/trunk@175584 bbb929c8-8fbe-4397-9dbb-9b2b20218538