Simplify time tracking in SMILTimeContainer
Instead of 5 difference time fields, use two - one to track the last
seek/pause time in the container ("presentation time"), and one to
track the document time corresponding to that.
Use two bool flags for tracking 'paused' and 'started' state.
Also straighten out code-flow in SMILTimeContainer::begin() to make it
a bit more obvious that we're essentially mirroring the contents of
updateAnimationsAndScheduleFrameIfNeeded. begin() is also renamed into
start(). Pass double to SMILTimeContainer::scheduleAnimationFrame, do
some ASSERT->DCHECK transformations when touching code and touch up