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Starts shelf layout manager shutdown earlier at the beginning of shell
shutdown sequence (during OnShellDestroying call issued to shell
observers when the shell tear down starts).

The main goal is to prevent shelf state updates that might happen during
shutdown. For example, tablet mode controller destruction, and overview
controller destruction might cause window stacking changes, that in
turn can cause shelf state updates - both of which happen before
Shell::CloseAllRootWindowChildWindows(), which is from where
ShelfLayoutManager::PrepareForShutdown is currently called (with some
indirection).

NOTE: Calls to start shelf shutdown issued from
RootWindowController::CloseChildWindows() are kept around to handle root
window controller shutdowns that happen outside shell shutdown.

BUG=1033343

Change-Id: I86211392b45bdafd5f095bd60c774fe88e8accce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015358Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734253}