Bug 1361915 - Record telemetry each time we try to run a transform animation on the compositor. r=birtles,bsmedberg

The telemetry is recorded once per effect:target pair, and is intended for
comparison with the telemetry added in bug 1349808.

MozReview-Commit-ID: 8JYbAifjmki

dom/animation/KeyframeEffect.cpp

@@ -134,6 +134,7 @@ KeyframeEffect::SetTarget(const Nullable<ElementOrCSSPseudoElement>& aTarget)
   // too so we have a more complete picture of the type of frame sizes we
   // encounter, hence we reset the telemetry flag here.
   mRecordedContentTooLarge = false;
+  mRecordedFrameSize = false;
 }
 
 void