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.

ReactPerf.start();
    fn();
    ReactPerf.stop();
    
    ReactPerf.getWasted();
    ReactPerf.getInclusive();
    ReactPerf.getExclusive();
    ReactPerf.getOperations();
    return ReactPerf.getLastMeasurements();
  }
  it('should count no-op update as waste', function() {
    var container = document.createElement('div');
    ReactDOM.render(<App />, container);
    var measurements = measure(() => {
      ReactDOM.render(<App />, container);
    });