I've tested the new Perf tools on one of the screens in one app I'm working on and it works great :)

One thing I wanted also on the old one Perf tools was to be able to check whether the Perf tools are started or stopped, so basically exposing `ReactDebugTool`'s `isProfiling` variable. My use for that is that I have a keyboard shortcut (in development) bound to start/stop the Perf tools and currently I'm forced to maintain such a variable myself to know whether to start or stop (which gets out of sync if I start/stop the perf tools without the keyboard shortcut).

If you don't think it's worthwhile to expose `isProfiling` feel free to close this issue.

checkDebugID(debugID);
    if (__DEV__) {
      if (isProfiling && currentFlushNesting > 0) {
        warning(
          currentTimerType === timerType,
          'There is an internal error in the React performance measurement code. ' +
          'We did not expect %s timer to stop while %s timer is still in ' +
          'progress for %s instance. Please report this as a bug in React.',
          timerType,
          currentTimerType || 'no',
          (debugID === currentTimerDebugID) ? 'the same' : 'another'
        );
        currentFlushMeasurements.push({
          timerType,
          instanceID: debugID,
          duration: performanceNow() - currentTimerStartTime,
        });
        currentTimerStartTime = null;
        currentTimerDebugID = null;
        currentTimerType = null;
      }
    }
    emitEvent('onEndLifeCycleTimer', debugID, timerType);
  },
  onBeginReconcilerTimer(debugID, timerType) {