fix(core): markDirty() should only mark flags when really scheduling tick

[JiaLiPassion]

Close #39296

Fix an issue that markDirty() will not trigger change detection.

The case is for example we have the following component.

export class AppComponent implements OnInit {
  constructor(private router: Router) {}

  ngOnInit() {
    this.router.events
      .pipe(filter((e) => e instanceof NavigationEnd))
      .subscribe(() => ɵmarkDirty(this));
  }
}

export class CounterComponent implements OnInit, OnDestroy {
  ngOnInit() {
    this.countSubject.pipe(takeUntil(this.destroy)).subscribe((count) => {
      this.count = count;
      ɵmarkDirty(this);
    });
  }

Then the app navigate from AppComponent to CounterComponent,
so there are 2 markDirty() call at in a row.

The 1st call is from AppComponent when router changed, the
2nd call is from CounterComponent.ngOnInit().

And the markDirty()->scheduleTick() code look like this

function scheduleTick(rootContext, flags) {
    const nothingScheduled = rootContext.flags === 0 /* Empty */;
    rootContext.flags |= flags;
    if (nothingScheduled && rootContext.clean == _CLEAN_PROMISE) {
      rootContext.schedule(() => {
	...
        if (rootContext.flags & RootContextFlags.DetectChanges)
          rootContext.flags &= ~RootContextFlags.DetectChanges;
          tickContext();

        rootContext.clean = _CLEAN_PROMISE;
        ...
      });

So in this case, the 1st markDirty() will

1. set rootContext.flags = 1
2. before tickContext(), reset rootContext.flags = 0
3. inside tickContext(), it will call CounterComponent.ngOnint(),
   so the 2nd markDirty() is called.
4. and the 2nd scheduleTick is called, nothingScheduled is true,
   but rootContext.clean is not _CLEAN_PROMISE yet, since the 1st markDirty tick
   is still running.
5. So nowhere will reset the rootContext.flags.
6. then in the future, any other markDirty() call will not trigger the tick, since
   nothingScheduled is always false.

So nothingScheduled means no tick is scheduled, rootContext.clean === _CLEAN_PROMISE
means no tick is running.
So we should set the flags to rootContext only when no tick is scheudled or running.

[mhevery]

presubmit

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