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__) {
      if (!isProfiling) {
        return;
      }
      isProfiling = false;
      resetMeasurements();
    }
  },
  getFlushHistory() {
    if (__DEV__) {
      return flushHistory;
    }
  },
  onBeginFlush() {
    if (__DEV__) {
      currentFlushNesting++;
      resetMeasurements();
    }
    emitEvent('onBeginFlush');
  },
  onEndFlush() {