Make the timer in SMILTimeContainer more of an implementation detail

Add a few new methods to represent the different actions required:

 scheduleAnimationFrame(SMILTime fireTime)
 immediateAnimationFrame()
 cancelAnimationFrame()

and replace uses of m_timer.foo() and startTimer with the appropriate
method.

BUG=231576

Review URL: https://codereview.chromium.org/143403005

git-svn-id: svn://svn.chromium.org/blink/trunk@166497 bbb929c8-8fbe-4397-9dbb-9b2b20218538