Make BrowserAccessibilityManager inherit from WebContentsObserver

Before this CL, BrowserAccessibilityManager did not inherit
WebContentsObserver, and gets manually notified by WebContentsImpl when
navigation-related events take place. BrowserAccessibilityManager should
implement WebContentsObserver, so it can be automatically notified of
these things.

After this CL, BrowserAccessibilityManager inherits from WebContentsObserver,
however does not implement any navigation-related methods. This CL makes it
easier to land future changes (tracked by https://crbug.com/981271) that
transition BrowserAccessibilityManager to not being manually notified from
WebContentsImpl anymore.

Bug: 981271
Change-Id: Iaa7ae9d3f20b990dd4b370f41a0fff108cc6653c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672135
Commit-Queue: Dominic Farolino <[email protected]>
Reviewed-by: Matt Falkenhagen <[email protected]>
Reviewed-by: Nektarios Paisios <[email protected]>
Cr-Commit-Position: refs/heads/master@{#674770}
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 0a3d626..37e9397c 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -4327,6 +4327,8 @@
     display_cutout_host_impl_->DidFinishNavigation(navigation_handle);
 
   if (navigation_handle->HasCommitted()) {
+    // TODO(domfarolino, dmazzoni): Do this using WebContentsObserver. See
+    // https://crbug.com/981271.
     BrowserAccessibilityManager* manager =
         static_cast<RenderFrameHostImpl*>(
             navigation_handle->GetRenderFrameHost())
@@ -5819,8 +5821,8 @@
 
   // Notify accessibility that the user is navigating away from the
   // current document.
-  //
-  // TODO(dmazzoni): do this using a WebContentsObserver.
+  // TODO(domfarolino, dmazzoni): Do this using WebContentsObserver. See
+  // https://crbug.com/981271.
   BrowserAccessibilityManager* manager =
       frame_tree_node->current_frame_host()->browser_accessibility_manager();
   if (manager)