fix angular/components#21674

When setting `ngZoneRunCoalescing` to true, `onStable` is not emitted correctly.
the reason is before this commit, the code looks like this:

```
// application code call `ngZone.run()`
ngzone.run(() => {}); // step 1

// inside NgZone, in the OnInvoke hook, NgZone try to delay the checkStable()

function delayChangeDetectionForEvents(zone: NgZonePrivate) {
  if (zone.lastRequestAnimationFrameId !== -1) { // step 9
    return;
  }
  zone.lastRequestAnimationFrameId = zone.nativeRequestAnimationFrame.call(global, () => { // step 2
    if (!zone.fakeTopEventTask) {
      zone.fakeTopEventTask = Zone.root.scheduleEventTask('fakeTopEventTask', () => {
        zone.lastRequestAnimationFrameId = -1; // step 3
        updateMicroTaskStatus(zone); // step 4
        checkStable(zone); // step 6
      }, undefined, () => {}, () => {});
    }
    zone.fakeTopEventTask.invoke();
  });
  updatemicroTaskStatus(zone);
}

function updateMicroTaskStatus(zone: NgZonePrivate, ignoreCheckRAFId = false) {
  if (zone._hasPendingMicrotasks ||
      ((zone.shouldCoalesceEventChangeDetection || zone.shouldCoalesceRunChangeDetection) &&
       zone.lastRequestAnimationFrameId !== -1)) { // step 5
    zone.hasPendingMicrotasks = true;
  } else {
    zone.hasPendingMicrotasks = false;
  }
}

function checkStable(zone: NgZonePrivate) {
  if (zone._nesting == 0 && !zone.hasPendingMicrotasks && !zone.isStable) { // step 7
    try {
      zone._nesting++;
      zone.onMicrotaskEmpty.emit(null);
    ...
}

// application ref subscribe onMicroTaskEmpty
ngzone.onMicroTaskEmpty.subscribe(() => {
  ngzone.run(() => { // step 8
    tick();
  });
});

```

and the process is:
1. step 1: application call ngZone.run()
2. step 2: NgZone delay the checkStable() call in a requestAnimationFrame, and also set
zone.lastRequestAnimationFrameId
3. step 3: Inside the requestAnimationFrame callback, reset zone.lastRequestAnimationFrameId first
4. step 4: update microTask status
5, step 5: if zone.lastRequestAnimationFrameId is -1, that means no microTask pending.
6. step 6: checkStable and trigger onMicrotaskEmpty emitter.
7. step 7: ApplicationRef subscribed onMicrotaskEmpty, so it will call another `ngZone.run()` to process
tick()
8. step 8: And this new `ngZone.run()` will try to check `zone.lastRequestAnimationFrameId` in `step 9`
when trying to delay the checkStable(), and since the zone.lastRequestAnimationFrameId is already reset
to -1 in step 3, so this ngZone.run() will run into step 2 again.
9. and become a infinite loop..., so onStable is never emit

in this commit, there is a new flag `zone.isCheckStableRunning` added to
prevent re-entry when `shouldCoaleascing` flag is enabled.