Move core/platform/graphics/UnitBezier to platform/animation.

This file is used only by animation code, so moving it to the correct
new directory. As a side-effect, creates the platform/animation
directory.

R=abarth@chromium.org, abarth, eae
BUG=303332

Review URL: https://codereview.chromium.org/25702006

git-svn-id: svn://svn.chromium.org/blink/trunk@158990 bbb929c8-8fbe-4397-9dbb-9b2b20218538