In Web Animations specification, if the animation effect is in none of the any phase, it define as 'Idle' phase. [1]
But, in the gecko, it defined as 'Null' phase.
So we will need to use 'Idle' phase for consistency.
[1] https://w3c.github.io/web-animations/#idle-phase
MozReview-Commit-ID: FlPpYOqdM4X
Before we begin re-arranging KeyframeEffect.h we move ComputedTiming aside
since putting it in a separate file should make navigating the source
easier.
MozReview-Commit-ID: L5GTFAo00sh
