The Animator.animate() function tests if (this._frameCount == this._dropFrameCount) before incrementing this._frameCount, causing it to store the wrong this._measureTimeOffset. This causes it to report 90fps when it's actually doing 60fps.
Created attachment 264367 [details] Patch
unofficial r=me.
Comment on attachment 264367 [details] Patch Clearing flags on attachment: 264367 Committed r191768: <http://trac.webkit.org/changeset/191768>
All reviewed patches have been landed. Closing bug.