Bug 1033114 Part 1: Update AnimationPlayer to spec changes r=birtles, r=bz
Bug 1033114 Part 2: Rename mPauseStart to mHoldTime to match spec r=birtles
Bug 1033114 Part 3: Make mStartTime a nullable TimeDuration r=birtles
Bug 1033114 Part 4: Make mStartTime a nullable TimeDuration r=birtles