Suppress redundant scheduling of frames while animating

While servicing animations, any style invalidations generated will be
handled in the same frame.

BUG=340679

Review URL: https://codereview.chromium.org/147993007

git-svn-id: svn://svn.chromium.org/blink/trunk@166800 bbb929c8-8fbe-4397-9dbb-9b2b20218538