ui: Add more class docs for AnimationMetricsRecorder/Reporter.

As requested in [1]. Idea is make it less needed to code-search for
examples.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2068305/10/ui/compositor/animation_metrics_recorder.h#16

Test: none
Bug: 1051490
Change-Id: I4a8bb4bc04e8f745913f215952daab90a54deafc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097236
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749250}

ui/compositor/animation_metrics_recorder.h

@@ -13,6 +13,26 @@ namespace ui {
 
 class AnimationMetricsReporter;
 
+// This is the interface to send animation smoothness numbers to a given
+// AnimationMetricsReporter.
+//
+// Classes that run animations (e.g. ui::LayerAnimationSequence,
+// views::CompositorAnimationRunner) should own one of these objects and pass
+// the values required to calculate animation smoothness when an animation
+// starts and ends.
+//
+// To use, when your animation starts:
+//   animation_metrics_recorder_->OnAnimationStart(frame, start_time,
+//       expected_duration);
+// and when it ends:
+//   animation metrics_recorder_->OnAnimationEnd(frame, refresh_rate);
+// and your attached AnimationMetricsReporter will report the calculated
+// smoothness. Note that if the animator is attached or detached during an
+// animation, this class will have to be notified.
+//
+// Unless implementing a complex custom animator, client code should just need
+// to supply an AnimationMetricsReporter to an animations class that already
+// owns an instance of this class.
 class COMPOSITOR_EXPORT AnimationMetricsRecorder {
  public:
   explicit AnimationMetricsRecorder(AnimationMetricsReporter* reporter);

ui/compositor/animation_metrics_reporter.h

@@ -9,9 +9,13 @@
 
 namespace ui {
 
+// Override this class and attach it to any class that supports recording
+// animation smoothness (e.g. ui::LayerAnimationSequence,
+// views::CompositorAnimationRunner). When an animation ends, |Report| will be
+// called with the animation smoothness as a percentage.
 class COMPOSITOR_EXPORT AnimationMetricsReporter {
  public:
-  virtual ~AnimationMetricsReporter() {}
+  virtual ~AnimationMetricsReporter() = default;
   // Called at the end of every animation sequence, if the duration and frames
   // passed meets certain criteria. |value| is the smoothness, measured in
   // percentage of the animation.