flackr@ from a threaded-rendering perspective, I assume you'd like to know which CSS properties are animated at all - independently of whether that mechanism is CSS animations or WebAnim, right?

suzy, would you generally also want the combined data?  Eg. I assume the fast-paths we implement (like for transform animation) are the same code regardless of whether the input was CSS or WebAnim, is that right?

See also discussion in https://codereview.chromium.org/2801003003/

Also I guess there's the question of whether we want to change the semantics of an existing UMA stat.  I believe WebAnim usage is still pretty low (Element.animate around 0.1% of page views), so this shouldn't change the values much.  So I'd argue we should just consider this a bug in the metric and fix the existing metric (assuming there's not some compelling reason to separate it out).  We'll want to update the description in histograms.xml when fixing this.

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