accessibility: Stop observing the tablet mode controller during shutdown

The present CL addresses the problem that AccessibilityControllerImpl
never actually removes itself as an observer of TabletModeController. In
the following part of the destructor, the "if" condition is always false
because the tablet mode controller is destroyed first:

  if (Shell::Get()->tablet_mode_controller())
    Shell::Get()->tablet_mode_controller()->RemoveObserver(this);

Change-Id: Ib35b19751862921691c767520e6803007c763a8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816906Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698624}

ash/accessibility/accessibility_controller_impl.cc

@@ -262,11 +262,7 @@ AccessibilityControllerImpl::AccessibilityControllerImpl()
   Shell::Get()->tablet_mode_controller()->AddObserver(this);
 }
 
-AccessibilityControllerImpl::~AccessibilityControllerImpl() {
-  if (Shell::Get()->tablet_mode_controller())
-    Shell::Get()->tablet_mode_controller()->RemoveObserver(this);
-  Shell::Get()->session_controller()->RemoveObserver(this);
-}
+AccessibilityControllerImpl::~AccessibilityControllerImpl() = default;
 
 // static
 void AccessibilityControllerImpl::RegisterProfilePrefs(
@@ -359,6 +355,11 @@ void AccessibilityControllerImpl::RegisterProfilePrefs(
   // TODO(jamescook): Move ownership to ash.
 }
 
+void AccessibilityControllerImpl::Shutdown() {
+  Shell::Get()->tablet_mode_controller()->RemoveObserver(this);
+  Shell::Get()->session_controller()->RemoveObserver(this);
+}
+
 void AccessibilityControllerImpl::SetHighContrastAcceleratorDialogAccepted() {
   if (!active_user_prefs_)
     return;

ash/accessibility/accessibility_controller_impl.h

@@ -56,6 +56,8 @@ class ASH_EXPORT AccessibilityControllerImpl : public AccessibilityController,
   // See Shell::RegisterProfilePrefs().
   static void RegisterProfilePrefs(PrefRegistrySimple* registry, bool for_test);
 
+  void Shutdown();
+
   void AddObserver(AccessibilityObserver* observer);
   void RemoveObserver(AccessibilityObserver* observer);
 

ash/shell.cc

@@ -665,6 +665,10 @@ Shell::~Shell() {
   // dependency on the latter.
   assistant_controller_.reset();
 
+  // Because this function will call |TabletModeController::RemoveObserver|, do
+  // it before destroying |tablet_mode_controller_|.
+  accessibility_controller_->Shutdown();
+
   // Destroy tablet mode controller early on since it has some observers which
   // need to be removed.
   tablet_mode_controller_->Shutdown();