Re-land r158052 after removing ASSERT_NOT_REACHED that caused crash.

Revert "Revert "Refactoring animation code in accelerated path.""

r158052 introduced invaild ASSERT_NOT_REACHED that caused crash. This CL re-land
r158052 after removing invalid ASSERT_NOT_REACHED.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/24409003

git-svn-id: svn://svn.chromium.org/blink/trunk@158315 bbb929c8-8fbe-4397-9dbb-9b2b20218538