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The present CL fixes what happens if you switch to tablet mode when the
following criteria are met:
1. There are at least two displays.
2. There is only one window.
3. The window is active.
4. The window is snapped on the primary display.

Without the present CL, NOTREACHED is hit (Issue 1024309), and there is
also a crash if DCHECKs are disabled (Issue 1024325).

Fixed: 1024309, 1024325
Change-Id: Ie1a6e378067076c7dbd8ec3ed388e70cd9382611
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1938463
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721061}