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.

if (__DEV__) {
      resetMeasurements();
      currentFlushNesting--;
    }
    emitEvent('onEndFlush');
  },
  onBeginLifeCycleTimer(debugID, timerType) {
    checkDebugID(debugID);
    emitEvent('onBeginLifeCycleTimer', debugID, timerType);
    if (__DEV__) {
      if (isProfiling && currentFlushNesting > 0) {
        warning(
          !currentTimerType,
          'There is an internal error in the React performance measurement code. ' +
          'Did not expect %s timer to start while %s timer is still in ' +
          'progress for %s instance.',
          timerType,
          currentTimerType || 'no',
          (debugID === currentTimerDebugID) ? 'the same' : 'another'
        );
        currentTimerStartTime = performanceNow();
        currentTimerDebugID = debugID;
        currentTimerType = timerType;
      }
    }
  },
  onEndLifeCycleTimer(debugID, timerType) {