Attributing a dropped frame to a reason

[sadrulhc]

The smoothness proposal makes it possible to detect dropped frames. This is helpful to measure the smoothness of animations/scrolls, however, these are not directly actionable. The API could expose some information about the dropped frames that would be more actionable for the web-developers (e.g. raf callback triggered expensive layout, etc.).

[sadrulhc]

The v1 proposal proposes a 'reason' field (arbitrary string) the browser may chose to include. It would be difficult to attribute a single reason to a dropped frame, so perhaps an array of possible reasons would be more appropriate.