Revert "Refactoring animation code in accelerated path."

We are currently crashing at an ASSERT_NOT_REACHED in
WebAnimationProvider.cpp(61). Reverting while we investigate.

This reverts commit e283c73dcea8ef7f5f4a9d4634cd168d5ef9a97f.

TBR=dongseong.hwang
BUG=None

Review URL: https://codereview.chromium.org/24239011

git-svn-id: svn://svn.chromium.org/blink/trunk@158137 bbb929c8-8fbe-4397-9dbb-9b2b20218538