Move core/platform/animation files

CSS/Style specific animation classes are moved to core/animation, the rest to platform/animation.

BUG=297477

Review URL: https://codereview.chromium.org/112463008

git-svn-id: svn://svn.chromium.org/blink/trunk@164710 bbb929c8-8fbe-4397-9dbb-9b2b20218538