Refactor content/renderer/accessibility to use AXTreeSerializer (re-land).

content/browser/renderer_host/render_view_host_impl.cc

Index: content/browser/renderer_host/render_view_host_impl.cc
diff --git a/content/browser/renderer_host/render_view_host_impl.cc b/content/browser/renderer_host/render_view_host_impl.cc
index f218d9758e0bbde8e785ae2a7530a1f7607a023c..35a45cf755ee667632f85f3af9ce4a70c8a9507c 100644
--- a/content/browser/renderer_host/render_view_host_impl.cc
+++ b/content/browser/renderer_host/render_view_host_impl.cc
@@ -1944,12 +1944,10 @@ void RenderViewHostImpl::OnAccessibilityEvents(
     const AccessibilityHostMsg_EventParams& param = params[i];
     if (static_cast<int>(param.event_type) < 0)
       continue;
-    ui::AXTreeUpdate update;
-    update.nodes = param.nodes;
     if (!ax_tree_)
-      ax_tree_.reset(new ui::AXTree(update));
+      ax_tree_.reset(new ui::AXTree(param.update));
     else
-      CHECK(ax_tree_->Unserialize(update)) << ax_tree_->error();
+      CHECK(ax_tree_->Unserialize(param.update)) << ax_tree_->error();
     accessibility_testing_callback_.Run(param.event_type);
   }
 }