Remove compositing trigger for will-change: top (and similar)

We can't use will-change: top (and similar) as a compositing trigger because it
changes the paint order by giving the RenderObject a RenderLayer of type
NormalLayer. That will cause us to paint incorrectly.

In a future CL, we could consider this property as a compositing hint if the
RenderObject already has a self-painting RenderLayer. However, let's first
restore correctness in the system before making that change.

R=esprehn@chromium.org
BUG=370604

Review URL: https://codereview.chromium.org/271753002

git-svn-id: svn://svn.chromium.org/blink/trunk@173574 bbb929c8-8fbe-4397-9dbb-9b2b20218538