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__) {
    eventHandlers.forEach(function(handler) {
      try {
        if (handler[handlerFunctionName]) {
          handler[handlerFunctionName](arg1, arg2, arg3, arg4, arg5);
        }
      } catch (e) {
        warning(
          handlerDoesThrowForEvent[handlerFunctionName],
          'exception thrown by devtool while handling %s: %s',
          handlerFunctionName,
          e.message
        );
        handlerDoesThrowForEvent[handlerFunctionName] = true;
      }
    });
  }
}
var isProfiling = false;
var flushHistory = [];
var currentFlushNesting = 0;
var currentFlushMeasurements = null;
var currentFlushStartTime = null;
var currentTimerDebugID = null;
var currentTimerStartTime = null;
var currentTimerType = null;
function clearHistory() {