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 handler2 = jasmine.createSpy('spy');
    var devtool1 = {onTestEvent: handler1};
    var devtool2 = {onTestEvent: handler2};
    ReactDebugTool.addDevtool(devtool1);
    ReactDebugTool.onTestEvent();
    expect(handler1.calls.length).toBe(1);
    expect(handler2.calls.length).toBe(0);
    ReactDebugTool.onTestEvent();
    expect(handler1.calls.length).toBe(2);
    expect(handler2.calls.length).toBe(0);
    ReactDebugTool.addDevtool(devtool2);
    ReactDebugTool.onTestEvent();
    expect(handler1.calls.length).toBe(3);
    expect(handler2.calls.length).toBe(1);
    ReactDebugTool.onTestEvent();
    expect(handler1.calls.length).toBe(4);
    expect(handler2.calls.length).toBe(2);
    ReactDebugTool.removeDevtool(devtool1);
    ReactDebugTool.onTestEvent();
    expect(handler1.calls.length).toBe(4);
    expect(handler2.calls.length).toBe(3);
    ReactDebugTool.removeDevtool(devtool2);
    ReactDebugTool.onTestEvent();
    expect(handler1.calls.length).toBe(4);
    expect(handler2.calls.length).toBe(3);
  });
  it('warns once when an error is thrown in devtool', () => {
    spyOn(console, 'error');
    ReactDebugTool.addDevtool({
      onTestEvent() {