Move CSSPropertyAnimation.{h,cpp} to core/animation/css

This allows the removal of the confusingly-named core/frame/animation directory.

R=dstockwell@chromium.org
BUG=334225

Review URL: https://codereview.chromium.org/164903002

git-svn-id: svn://svn.chromium.org/blink/trunk@167128 bbb929c8-8fbe-4397-9dbb-9b2b20218538