Reland "Fix double counting num_tracked_windows"

This reverts commit 9fb5b29e03bfb58b64746d20e1247ea7bb241ce0.

Reason for revert: the failed test requires DCHECK. Updated the test to skip if not enabled.

Bug: 435754476
Original change's description:
> Revert "Fix double counting num_tracked_windows"
>
> This reverts commit b1c9546fda0f18e517cc32a69ae5946a92151663.
>
> Reason for revert: The added test
> WindowOcclusionTrackerTest.DoNotCountTwice
> is failing on
> https://ci.chromium.org/ui/p/chromium/builders/ci/Linux%20Tests
> https://ci.chromium.org/ui/p/chromium/builders/ci/chromeos-amd64-generic-rel-gtest
>
> First failing builds:
> https://ci.chromium.org/ui/p/chromium/builders/ci/Linux%20Tests/165621/overview
> https://ci.chromium.org/ui/p/chromium/builders/ci/chromeos-amd64-generic-rel-gtest/26527/overview
>
> Bug: 435754476
> Original change's description:
> > Fix double counting num_tracked_windows
> >
> > WindowOcclusionTracker (WOT) may count the window twice if the Tracking
> > starts in WindowObserver::OnParentChanged (there may be other cases)
> > 1) It's increemented upon Track() because it has a root window. [1]
> > 2) Then incremented again in OnWindowAddedToRootWindow[2]
> >
> > This leave WOT in inconsistent state and leaves
> > WindowTreeHostObserver, which leads to CHECK failure later.
> >
> > This CL fixes by
> > 1) Observe WindowTreeHost (WTH) if not observed yet.
> > 2) Unobserve WHT if there is no tracked window that belongs to
> >    after a window is being removed from the tree.
> >
> > This CL keeps num_tracked_windows_ for verification purpose (assumes
> > existing code base do not have such use case). I'll remove it before
> > m142 branch cut unless this causes an issue.
> >
> > [1] https://source.chromium.org/chromium/chromium/src/+/main:ui/aura/window.cc;drc=54e79869abfe54e3f4db0298641659272f6f7ccb;l=563
> > [2] https://source.chromium.org/chromium/chromium/src/+/main:ui/aura/window.cc;drc=54e79869abfe54e3f4db0298641659272f6f7ccb;l=567
> >
> > Bug: 435754476
> > Test: covered by unittest.
> > Change-Id: Ia9e4fb3ae0a2aabae1f291de94e8130d8d174f4c
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6871198
> > Reviewed-by: Zoraiz Naeem <zoraiznaeem@chromium.org>
> > Reviewed-by: Eliot Courtney <edcourtney@chromium.org>
> > Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
> > Cr-Commit-Position: refs/heads/main@{#1509922}
>
> Bug: 435754476
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Change-Id: Iee15e1a60abb3ecf51fc65b881c8e34473b5bf1c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6909978
> Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Auto-Submit: Hiroshige Hayashizaki <hiroshige@chromium.org>
> Owners-Override: Hiroshige Hayashizaki <hiroshige@chromium.org>
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Cr-Commit-Position: refs/heads/main@{#1509947}

Bug: 435754476
Change-Id: I252bfd3529bc774a13f86e19601f6accf3d86526
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6909948
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Zoraiz Naeem <zoraiznaeem@chromium.org>
Reviewed-by: Eliot Courtney <edcourtney@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1510110}