This augments the AnimationMetricsTracker to also track compositor animations
triggered by chrome and content layers separately. During the animation, the
tracker keeps a count of frames composited, and once the animation ends, it
uses the wall-clock time and vsync interval to compute the expected number of
composited frames. It then submits a ratio of actual/expected to telemetry.
A score of 1000 (because the ratio is scaled up to an integer between 0 and 1000)
indicates a perfect score with no frames dropped. Lower values are worse, and
values significantly above 1000 indicate abnormal behaviour. Values may be slightly
above 1000 due to rounding error or vsync jitter.
MozReview-Commit-ID: 30Vw0j3dm9G
