Fix invalid references to QuickAnswersPreTargetHandler

QuickAnswersPreTargetHandler is added as a ViewObserver to the
QuickAnswersView. Since the view owns an instance of the handler, it
must stop observing the view when it is deleted to prevent invalid
references to it from ~QuickAnswersView() later.

The issue was observed when a failing trybot reverted a related change
in chromium/2212238.

is_debug set to true.

Bug: b:157182213
Test: Tested on Chrome OS VM and running ash:ash_unittests with
Change-Id: If59e512a45faab65a99ed4e76a1e00c5e60550aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216816Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Abhijeet Singh <siabhijeet@google.com>
Cr-Commit-Position: refs/heads/master@{#772430}

ash/quick_answers/ui/quick_answers_pre_target_handler.cc

@@ -35,11 +35,11 @@ void QuickAnswersPreTargetHandler::Init() {
   // mouse capture as well as a pre-target handler, so we need to register one
   // here as well to intercept events for QuickAnswersView.
   Shell::Get()->AddPreTargetHandler(this, ui::EventTarget::Priority::kSystem);
-  view_->AddObserver(this);
+  view_observer_.Add(view_);
 }
 
 void QuickAnswersPreTargetHandler::OnEvent(ui::Event* event) {
-  if (!event->IsLocatedEvent())
+  if (!view_observer_.IsObservingSources() || !event->IsLocatedEvent())
     return;
 
   // Clone event to forward down the view-hierarchy.
@@ -94,7 +94,7 @@ void QuickAnswersPreTargetHandler::OnViewIsDeleting(
     if (active_menu_instance)
       active_menu_instance->Cancel(views::MenuController::ExitType::kAll);
   }
-  view_->RemoveObserver(this);
+  view_observer_.Remove(view_);
 }
 
 // TODO(siabhijeet): Investigate using SendEventsToSink() for dispatching.

ash/quick_answers/ui/quick_answers_pre_target_handler.h

@@ -5,17 +5,15 @@
 #ifndef ASH_QUICK_ANSWERS_UI_QUICK_ANSWERS_PRE_TARGET_HANDLER_H_
 #define ASH_QUICK_ANSWERS_UI_QUICK_ANSWERS_PRE_TARGET_HANDLER_H_
 
+#include "base/scoped_observer.h"
 #include "ui/events/event_handler.h"
+#include "ui/views/view.h"
 #include "ui/views/view_observer.h"
 
 namespace ui {
 class LocatedEvent;
 }  // namespace ui
 
-namespace views {
-class View;
-}  // namespace views
-
 namespace ash {
 
 class QuickAnswersView;
@@ -58,6 +56,7 @@ class QuickAnswersPreTargetHandler : public ui::EventHandler,
 
   // Associated view handled by this class.
   views::View* const view_;
+  ScopedObserver<views::View, views::ViewObserver> view_observer_{this};
 
   // Whether any active menus, |view_| is a companion Quick-Answers related view
   // of which, should be dismissed when it is deleted.