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Remove Ash's separate registration of root windows for SPM.
Remove UAF and root window determination workarounds for SPM.

Bug: 958471
Test: No ui_devtools regressions
Change-Id: I55fe07b83731c3832e803078eb75526972534958
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1631904Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664356}