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This change has the clipboard nudge observe the hotseat state and will
animate the bounds of the clipboard nudge so that the nudge is always
placed above the hotseat.

Also, because the ClipboardNudge uses RootWindows the
clipboard_history_controller_ must be reset in before the
window_tree_host_manager_ is shutdown. This fixes the original CL which
was reverted due to msan issues.

Bug: 1137019
Change-Id: Ibbe8f06132f51d569a5ea4993df77ef042eabeb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2477524
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818547}