Fix scheduleAnimation for WebPagePopups.

This should tell the compositor about it if a compositor is being used.

BUG=372278

Review URL: https://codereview.chromium.org/290573006

git-svn-id: svn://svn.chromium.org/blink/trunk@174114 bbb929c8-8fbe-4397-9dbb-9b2b20218538