(Regarding the experimental `React.unstable_Profiler` API and reactjs/rfcs/pull/51.)

PR #13058 changed the behavior of React profiling builds to opt host roots into `ProfileMode` if the React DevTools extension is detected. This was necessary in order to have accurate tree base time measurements for the visual flame graphs.

The Profiler plug-in for DevTools does feature detection to determine if it's enabled by looking at each root and seeing if any of the fibers contain an `actualDuration` attribute,

The problem is that the `actualDuration` attribute will be present on v16.4 fibers, which pre-date PR #13058. As a result, the Profiler UI may enable itself with versions of React that it cannot work correctly with.

Perhaps an easy solution would be to rename one of the `unstable_Profiler`'s fiber attributes prior to the next minor release (e.g. `treeBaseTime` -> `treeBaseDuration`) and change the DevTools Profiler to feature check using this new name instead.
if (!enableProfilerTimer) {
    return;
  }
  if (isProcessingSyncWork && syncCommitStartTime === 0) {
    syncCommitStartTime = now();
  }
  if (timerPausedAt > 0) {
    totalElapsedPauseTime += now() - timerPausedAt;
    timerPausedAt = 0;
  }
}
function resetActualRenderTimerStackAfterFatalErrorInDev(): void {
  if (!enableProfilerTimer) {
    return;
  }
  if (__DEV__) {
    fiberStack.length = 0;
  }
}
/**
 * The "base" render time is the duration of the “begin” phase of work for a particular fiber.
 * This time is measured and stored on each fiber.
 * The time for all sibling fibers are accumulated and stored on their parent during the "complete" phase.
 * If a fiber bails out (sCU false) then its "base" timer is cancelled and the fiber is not updated.
 */
let baseStartTime: number = -1;
function recordElapsedBaseRenderTimeIfRunning(fiber: Fiber): void {