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 n = Math.pow(10, base);
  return Math.floor(val * n) / n;
}
function getFlushHistory() {
  return ReactDebugTool.getFlushHistory();
}
function getExclusive(flushHistory = getFlushHistory()) {
  var aggregatedStats = {};
  var affectedIDs = {};
  function updateAggregatedStats(treeSnapshot, instanceID, timerType, applyUpdate) {
    var {displayName} = treeSnapshot[instanceID];
    var key = displayName;
    var stats = aggregatedStats[key];
    if (!stats) {
      affectedIDs[key] = {};
      stats = aggregatedStats[key] = {
        key,
        instanceCount: 0,
        counts: {},
        durations: {},
        totalDuration: 0,
      };
    }
    if (!stats.durations[timerType]) {
      stats.durations[timerType] = 0;
    }