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 type {Lane, Lanes} from './ReactFiberLane';
import type {Fiber} from './ReactInternalTypes';
import type {Wakeable} from 'shared/ReactTypes';
import {enableSchedulingProfiler} from 'shared/ReactFeatureFlags';
import ReactVersion from 'shared/ReactVersion';
import getComponentName from 'shared/getComponentName';
/**
 * If performance exists and supports the subset of the User Timing API that we
 * require.
 */
const supportsUserTiming =
  typeof performance !== 'undefined' && typeof performance.mark === 'function';
function formatLanes(laneOrLanes: Lane | Lanes): string {
  return ((laneOrLanes: any): number).toString();
}
if (enableSchedulingProfiler) {
  if (supportsUserTiming) {