Make the `enableSchedulingProfiler` flag dynamic for the profiling build (so marks can be disabled). I think it should be sufficient to do something like:
```js
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.)
/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */
import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
import typeof * as ExportsType from './ReactFeatureFlags.www';
import typeof * as DynamicFeatureFlags from './ReactFeatureFlags.www-dynamic';
const dynamicFeatureFlags: DynamicFeatureFlags = require('ReactFeatureFlags');
export const {
  debugRenderPhaseSideEffectsForStrictMode,
  disableInputAttributeSyncing,
  enableTrustedTypesIntegration,
  disableSchedulerTimeoutBasedOnReactExpirationTime,
  warnAboutSpreadingKeyToJSX,
  replayFailedUnitOfWorkWithInvokeGuardedCallback,
  enableFilterEmptyStringAttributesDOM,
  enableLegacyFBSupport,
  deferRenderPhaseUpdateToNextBatch,
  decoupleUpdatePriorityFromScheduler,
  enableDebugTracing,
  enableDoubleInvokingEffects,
  enableUseRefAccessWarning,
} = dynamicFeatureFlags;
export const enableProfilerTimer = __PROFILE__;