Scheduling profiler tweaks

[bvaughn]

Make the enableSchedulingProfiler flag dynamic for the profiling build (so marks can be disabled). I think it should be sufficient to do something like:

export const enableSchedulingProfiler =
  __PROFILE__ && dynamicFeatureFlags.enableSchedulingProfiler;

Update our User Timing API check to include clearMeasures and when we call mark(name), also call clearMeasures(name) to avoid clogging the entries buffer. (Apparently this has a significant negative performance impact for Firefox.)