Replace ScopedObserver with base::ScopedObservation in /content/browser/accessibility.

ScopedObserver is being deprecated in favor of two new classes:
- base::ScopedObservation for observers that only ever observe
  a single source.
- base::ScopedMultiSourceObservation for observers that do or may
  observe more than a single source.
This CL was uploaded by git cl split.

R=jdiggs@igalia.com

Bug: 1145565
Change-Id: I7c12763d1b3461e3d66280174dc6164c46b63c1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526902Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826447}

content/browser/accessibility/browser_accessibility_manager.cc

@@ -160,7 +160,7 @@ BrowserAccessibilityManager::BrowserAccessibilityManager(
       use_custom_device_scale_factor_for_testing_(false),
       tree_(std::make_unique<ui::AXSerializableTree>()),
       event_generator_(ax_tree()) {
-  tree_observer_.Add(ax_tree());
+  tree_observation_.Observe(ax_tree());
 }
 
 BrowserAccessibilityManager::BrowserAccessibilityManager(
@@ -175,7 +175,7 @@ BrowserAccessibilityManager::BrowserAccessibilityManager(
       use_custom_device_scale_factor_for_testing_(false),
       tree_(std::make_unique<ui::AXSerializableTree>()),
       event_generator_(ax_tree()) {
-  tree_observer_.Add(ax_tree());
+  tree_observation_.Observe(ax_tree());
   Initialize(initial_tree);
 }
 

content/browser/accessibility/browser_accessibility_manager.h

@@ -16,7 +16,7 @@
 
 #include "base/callback_forward.h"
 #include "base/macros.h"
-#include "base/scoped_observer.h"
+#include "base/scoped_observation.h"
 #include "build/build_config.h"
 #include "content/browser/accessibility/accessibility_buildflags.h"
 #include "content/browser/accessibility/browser_accessibility_position.h"
@@ -612,7 +612,8 @@ class CONTENT_EXPORT BrowserAccessibilityManager : public ui::AXTreeObserver,
   // This member needs to be destructed before any observed AXTrees. Since
   // destructors for non-static member fields are called in the reverse order of
   // declaration, do not move this member above other members.
-  ScopedObserver<ui::AXTree, ui::AXTreeObserver> tree_observer_{this};
+  base::ScopedObservation<ui::AXTree, ui::AXTreeObserver> tree_observation_{
+      this};
 
   DISALLOW_COPY_AND_ASSIGN(BrowserAccessibilityManager);
 };