Revert "Prepare shelf layout manager for shutdown earlier"

This reverts commit 97dfeb90.

Reason for revert: TabletModeControllerTest.VerticalHingeTest (CRASHED) on Linux ChromiumOS MSan Tests
Fixed: 1044871

Original change's description:
> Prepare shelf layout manager for shutdown earlier
> 
> 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/+/2015358
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Commit-Queue: Toni Baržić <tbarzic@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#734253}

TBR=xiyuan@chromium.org,tbarzic@chromium.org

Change-Id: I69855c62eedfc7698df237169fc1282868d2876b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1033343
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015879Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734371}

ash/shelf/shelf_layout_manager.cc

@@ -427,11 +427,6 @@ void ShelfLayoutManager::InitObservers() {
 }
 
 void ShelfLayoutManager::PrepareForShutdown() {
-  // This might get called twice during shell shutdown - once at the beginning,
-  // and once as part of root window controller shutdown.
-  if (in_shutdown_)
-    return;
-
   in_shutdown_ = true;
 
   // Stop observing changes to avoid updating a partially destructed shelf.
@@ -919,13 +914,6 @@ void ShelfLayoutManager::OnPinnedStateChanged(aura::Window* pinned_window) {
   UpdateVisibilityState();
 }
 
-void ShelfLayoutManager::OnShellDestroying() {
-  // Prepare for shutdown early, to prevent window stacking changes that may
-  // happen during shutdown (e.g. during overview controller, or tablet mode
-  // controller destruction) from causing visibility and state updates.
-  PrepareForShutdown();
-}
-
 void ShelfLayoutManager::OnSplitViewStateChanged(
     SplitViewController::State previous_state,
     SplitViewController::State state) {

ash/shelf/shelf_layout_manager.h

@@ -187,7 +187,6 @@ class ASH_EXPORT ShelfLayoutManager
   void OnShelfAutoHideBehaviorChanged(aura::Window* root_window) override;
   void OnUserWorkAreaInsetsChanged(aura::Window* root_window) override;
   void OnPinnedStateChanged(aura::Window* pinned_window) override;
-  void OnShellDestroying() override;
 
   // SplitViewObserver:
   void OnSplitViewStateChanged(SplitViewController::State previous_state,