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1) The Home Launcher used to use the wrong display ID when in
   Unified Desktop mode.
2) If (1) is fixed, we hit https://crbug.com/902601. The captive
   portal dialog widget used to be leaked and never destroyed.
3) If (2) is fixed, we crash on the first attempt to press the
   app list button. The reason is tablet mode triggers a switch
   to mirror mode. This switch happens asynchronously after the
   Home Launcher had already been created. Switching from Unified
   to mirror mode destroys the Unified host and the Home Launcher.
   That's why we need to ensure that the Home Launcher is
   recreated.

BUG=900956, 902601
TEST=Added a test that crashes without the fix.

Change-Id: If6eb9c2255dfa9d442aa115a3274db2d8a4110d7
Reviewed-on: https://chromium-review.googlesource.com/c/1325389
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Weidong Guo <weidongg@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606687}