Refactor SMILTimeContainer can-schedule-frame predicate

begin() and updateAnimationsAndScheduleFrameIfNeeded() use the same
predicate to check if they should schedule an animation frame - although
they phrase it slightly differently (because of local knowledge.)
Move the generic version to a canScheduleFrame() method and use that in
both cases.

BUG=631879

Review-Url: https://codereview.chromium.org/2257803002
Cr-Commit-Position: refs/heads/master@{#413328}
2 files changed