Reland: Marks the document object as busy while it is still loading

Changes in the reland compared to the original patch:
Switched to using GetRoot instead of Root() which might prevent the use of an uninitialized root object.

Original description:
Screen readers want to have a reliable way to know whether a document is still loading so as to ignore events coming from it.
The current change can be made irrespective to whether we will mark the document as loaded when HTML parsing has started or when the "interactive" readyState has been entered.

R=dmazzoni@chromium.org, aleventhal@chromium.org

Bug: 897177, 903402
Change-Id: I681883999918ffdd32fc11d2f691b659c7e91149
Reviewed-on: https://chromium-review.googlesource.com/c/1327745
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607017}

content/browser/accessibility/browser_accessibility_manager.cc

@@ -207,8 +207,7 @@ void BrowserAccessibilityManager::FireFocusEventsIfNeeded() {
   // or for the document to actually finish loading.
   if (focus && focus == focus->manager()->GetRoot() &&
       focus->PlatformChildCount() == 0 &&
-      !focus->GetBoolAttribute(ax::mojom::BoolAttribute::kBusy) &&
-      !focus->manager()->GetTreeData().loaded) {
+      focus->GetBoolAttribute(ax::mojom::BoolAttribute::kBusy)) {
     focus = nullptr;
   }
 

content/renderer/accessibility/blink_ax_tree_source.cc

@@ -443,6 +443,9 @@ void BlinkAXTreeSource::SerializeNode(WebAXObject src,
   TRACE_EVENT1("accessibility", "BlinkAXTreeSource::SerializeNode", "role",
                ui::ToString(dst->role));
 
+  if (src.Equals(GetRoot()) && !GetRoot().IsLoaded())
+    dst->AddBoolAttribute(ax::mojom::BoolAttribute::kBusy, true);
+
   WebAXObject offset_container;
   WebFloatRect bounds_in_container;
   SkMatrix44 container_transform;
@@ -451,7 +454,7 @@ void BlinkAXTreeSource::SerializeNode(WebAXObject src,
                         container_transform, &clips_children);
   dst->location = bounds_in_container;
 #if !defined(OS_ANDROID) && !defined(OS_MACOSX)
-  if (src.Equals(root())) {
+  if (src.Equals(GetRoot())) {
     WebView* web_view = render_frame_->GetRenderView()->GetWebView();
     std::unique_ptr<gfx::Transform> container_transform_gfx =
         std::make_unique<gfx::Transform>(container_transform);