BrowserAccessibility should own a AXPlatformNode.

content/browser/accessibility/browser_accessibility.h

Index: content/browser/accessibility/browser_accessibility.h
diff --git a/content/browser/accessibility/browser_accessibility.h b/content/browser/accessibility/browser_accessibility.h
index cbadc843834ccd209a85d680d47d913ad8e17cd4..37351c7e9931f7aca2240cc7ddb7bfb97062865d 100644
--- a/content/browser/accessibility/browser_accessibility.h
+++ b/content/browser/accessibility/browser_accessibility.h
@@ -393,6 +393,9 @@ class CONTENT_EXPORT BrowserAccessibility : public ui::AXPlatformNodeDelegate {
   // A unique ID, since node IDs are frame-local.
   int32_t unique_id_;
 
+  // The ui platform backend

[dmazzoni, 2017/03/21 16:45:00]

nit: Make comments full sentences, end with a period.

I'd make this comment a bit more detailed so it's more clear what we're
doing here during the transition period. Something like:

// The platform-specific object that implements the accessibility APIs for this
node.
// Currently some of the platform-specific code is implemented by subclasses of
// BrowserAccessibility and some by |platform_node_|, but eventually we want all
// of that code to be in AXPlatformNode.  http://crbug.com/xxxxxx

+  ui::AXPlatformNode* platform_node_;
+
  private:
   // |GetInnerText| recursively includes all the text from descendants such as
   // text found in any embedded object. In contrast, |GetText| might include a