Frozen animations still contribute to the sandwich/animation stack

With (for instance) a to-animation, the frozen element will for instance
provide the 'underlying value' to any following element.

BUG=374793

Review URL: https://codereview.chromium.org/316013003

git-svn-id: svn://svn.chromium.org/blink/trunk@175530 bbb929c8-8fbe-4397-9dbb-9b2b20218538