wm: Update nested activation handling

- NOTREACHED() and no-ope if the nested activation wants to change
  to a different active window other than the one being activated;
- Allow focus shift in nested activations as long as they don't
  change the window to be activated

Bug: 908524
Test: FocusControllerApiTest.ActivateWhileActivating
Change-Id: I105c85bf2671c7e396fdcbedff7b8082f007f642
Reviewed-on: https://chromium-review.googlesource.com/c/1357017
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Mitsuru Oshima (gardener - slow) <oshima@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616346}
3 files changed