Animations: Make perf tests for web animations animate properly.

Web Animations uses ms for duration, while legacy keyframe animations uses sec
for duration. Currently, the given perf tests use sec, so animations are weird.

BUG=309981

Review URL: https://codereview.chromium.org/433423003

git-svn-id: svn://svn.chromium.org/blink/trunk@180150 bbb929c8-8fbe-4397-9dbb-9b2b20218538