Bug 1082098 - Part 1 - Return correct change hint when scroll-behavior CSS value changes. r=roc
- When the scroll-behavior CSS value changed, the nsChangeHint_NeutralChange
  hint was returned by nsStyleDisplay::CalcDifference.  It now returns
  nsChangeHint_ReconstructFrame to ensure that the change takes effect.
- When scroll-behavior is changed, the nsChangeHint_NeutralChange was not
  sufficient to enter nsCSSFrameConstructor::PropagateScrollToViewport.  By
  using the same hint as used when the overflow css property changes,
  nsChangeHint_ReconstructFrame, PropagateScrollToViewport will be called.
- The scroll-behavior css property is not expected to change often (the
  CSSOM-View DOM methods are likely to be used in those cases); however, if
  this does become common perhaps a faster-path might be worth while.
Bug 1082098 - Part 2 - Tests. r=roc
- scroll-behavior-8.html - Tests if dynamically changing the scroll-behavior
  css property on a div element takes effect after the page has been painted and
  reflowed.
- scroll-behavior-9.html - Tests if dynamically changing the scroll-behavior
  on the body element takes effect after the page has been painted and
  reflowed.
Bug 1088481 - stop progress timer in MediaDecoder::Shutdown if necessary. r=cpearce