Clip path shouldn't cause constant repaints

During style recalc, we were comparing the old and new clip path by pointer,
which meant we'd think we had a new clip path whenever we malloced up a new
clip path object. However, we don't need to trigger a repaint if the contents
of the path are the same as before.

R=eseidel@chromium.org
BUG=378948

Review URL: https://codereview.chromium.org/469943004

git-svn-id: svn://svn.chromium.org/blink/trunk@180598 bbb929c8-8fbe-4397-9dbb-9b2b20218538