Remove bogus ASSERT

This CL removes some bogus ASSERTs.  Now that we don't have the legacy
animation engine, we use these code paths all the time.

TBR=dstockwell@chromium.org
BUG=342801

Review URL: https://codereview.chromium.org/157523007

git-svn-id: svn://svn.chromium.org/blink/trunk@166934 bbb929c8-8fbe-4397-9dbb-9b2b20218538