Oilpan: Build fix for oilpan

Some change used PassRefPtrWillBeRawPtr for AnimatableColor which is not yet on the heap.
Thus build is broken in oilpan bots. This CL fixes it.

TBR=kouhei

Review URL: https://codereview.chromium.org/183183010

git-svn-id: svn://svn.chromium.org/blink/trunk@168574 bbb929c8-8fbe-4397-9dbb-9b2b20218538