Someone responded to Chromium's Issue 422000. Their response:
Assigning to self since Brandon already did his part on this bug.
+simonhong since the "unified BeginFrame for Aura" patch (https://codereview.chromium.org/1016033006) definitely added noise to the graph.
Depending on how bad the jitter is, maybe we should have the unified BeginFrame work land behind a flag while we work on making it more responsive and integrating it with the rest of the system.
After the patch above, the rAF start can be blocked by work on the Browser's UI thread which is where the jitter is coming from. This isn't necessarily bad though since going through the Browser's UI thread also allows us to avoid races with user input, which can actually improve perceived smoothness; but we haven't yet integrated with jdduke's work to get that benefit.
Unified BeginFrame should also become more responsive as we move work off the Browser's UI thread with efforts like impl-side-painting in the Browser. We should probably see if there's anything else monopolizing the UI thread and what we can do about it (blink scheduler on the UI thread?).