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__) {
    var previousStartTime = currentFlushStartTime;
    var previousMeasurements = currentFlushMeasurements || [];
    var previousOperations = ReactHostOperationHistoryDevtool.getHistory();
    if (!isProfiling || currentFlushNesting === 0) {
      currentFlushStartTime = null;
      currentFlushMeasurements = null;
      clearHistory();
      return;
    }
    if (previousMeasurements.length || previousOperations.length) {
      var registeredIDs = ReactComponentTreeDevtool.getRegisteredIDs();
      flushHistory.push({
        duration: performanceNow() - previousStartTime,
        measurements: previousMeasurements || [],
        operations: previousOperations || [],
        treeSnapshot: getTreeSnapshot(registeredIDs),
      });
    }
    clearHistory();
    currentFlushStartTime = performanceNow();
    currentFlushMeasurements = [];
  }
}
function checkDebugID(debugID) {