Views a11y: Implement AXPlatformNode::FromNativeViewAccessible on all platforms.

ui/views/accessibility/native_view_accessibility.h

Index: ui/views/accessibility/native_view_accessibility.h
diff --git a/ui/views/accessibility/native_view_accessibility.h b/ui/views/accessibility/native_view_accessibility.h
index ec1e0dbe626e0f256b985e9d75acd184515b5687..348bdbe12024c21dd7e3d48c4471fbb65a810364 100644
--- a/ui/views/accessibility/native_view_accessibility.h
+++ b/ui/views/accessibility/native_view_accessibility.h
@@ -42,6 +42,8 @@ class VIEWS_EXPORT NativeViewAccessibility
     : public ui::AXPlatformNodeDelegate,
       public WidgetObserver {
  public:
+  // A method for View to create a platform-specific NativeViewAccessibility
+  // instance for |view|. To retrieve the instance for |view|, use GetForView().
   static std::unique_ptr<NativeViewAccessibility> Create(View* view);
 
   ~NativeViewAccessibility() override;
@@ -75,6 +77,10 @@ class VIEWS_EXPORT NativeViewAccessibility
  protected:
   explicit NativeViewAccessibility(View* view);
 
+  // Retrieves the NativeViewAccessibility instance for the given View. This
+  // invokes View::GetNativeViewAccessible() to ensure it exists.
+  static NativeViewAccessibility* GetForView(View* view);
+
   // Weak. Owns this.
   View* view_;