[scheduler] Yield many times per frame, no rAF

Adds experimental flag to yield many times per frame using a message
event loop, instead of the current approach of guessing the next vsync
and yielding at the end of the frame.

This new approach forgoes a `requestAnimationFrame` entirely. It posts a
message event and performs a small amount of work (5ms) before yielding
to the browser, regardless of where it might be in the vsync cycle. At
the end of the event, if there's work left over, it posts another
message event.

This should keep the main thread responsive even for really high frame
rates. It also shouldn't matter if the hardware frame rate changes after
page load (our current heuristic only detects if the frame rate
increases, not decreases).

The main risk is that yielding more often will exacerbate main thread
contention with other browser tasks.

Let's try it and see.

packages/scheduler/src/SchedulerFeatureFlags.js

@@ -10,3 +10,4 @@ export const enableSchedulerDebugging = false;
 export const enableIsInputPending = false;
 export const requestIdleCallbackBeforeFirstFrame = false;
 export const requestTimerEventBeforeFirstFrame = false;
+export const enableMessageLoopImplementation = false;