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.

var {displayName, ownerID} = treeSnapshot[instanceID];
    var owner = treeSnapshot[ownerID];
    var key = (owner ? owner.displayName + ' > ' : '') + displayName;
    var stats = aggregatedStats[key];
    if (!stats) {
      affectedIDs[key] = {};
      stats = aggregatedStats[key] = {
        key,
        instanceCount: 0,
        inclusiveRenderDuration: 0,
        renderCount: 0,
      };
    }
    affectedIDs[key][instanceID] = true;
    applyUpdate(stats);
  }
  var isCompositeByID = {};
  flushHistory.forEach(flush => {
    var {measurements} = flush;
    measurements.forEach(measurement => {
      var {instanceID, timerType} = measurement;
      if (timerType !== 'render') {
        return;
      }
      isCompositeByID[instanceID] = true;
    });
  });
  flushHistory.forEach(flush => {
    var {measurements, treeSnapshot} = flush;
    measurements.forEach(measurement => {