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.

warning(debugID, 'ReactDebugTool: debugID may not be empty.');
}
var ReactDebugTool = {
  addDevtool(devtool) {
    eventHandlers.push(devtool);
  },
  removeDevtool(devtool) {
    for (var i = 0; i < eventHandlers.length; i++) {
      if (eventHandlers[i] === devtool) {
        eventHandlers.splice(i, 1);
        i--;
      }
    }
  },
  beginProfiling() {
    if (__DEV__) {
      if (isProfiling) {
        return;
      }
      isProfiling = true;
      flushHistory.length = 0;
      resetMeasurements();
    }
  },
  endProfiling() {