Migrate BrowserAccessibility windows unique id handling to AXPlatformNodeWin.

ui/accessibility/platform/ax_platform_node_win.h

Index: ui/accessibility/platform/ax_platform_node_win.h
diff --git a/ui/accessibility/platform/ax_platform_node_win.h b/ui/accessibility/platform/ax_platform_node_win.h
index e85a3c1855b40020fa5903b64fb273f9c1ef8c93..f6db0b8916f5a0ff7ab90468801a959936b602fe 100644
--- a/ui/accessibility/platform/ax_platform_node_win.h
+++ b/ui/accessibility/platform/ax_platform_node_win.h
@@ -277,6 +277,8 @@ class AX_EXPORT __declspec(uuid("26f5641a-246d-457b-a96d-07f3fae6acf2"))
 
   // Clear node's current relationships and set them to the default values.
   void CalculateRelationships();
+  static AXPlatformNode* GetFromUniqueId(int32_t unique_id);
+  int32_t unique_id() const { return unique_id_; }
 
   // AXPlatformNode overrides.
   gfx::NativeViewAccessible GetNativeViewAccessible() override;
@@ -650,6 +652,8 @@ class AX_EXPORT __declspec(uuid("26f5641a-246d-457b-a96d-07f3fae6acf2"))
   void Dispose() override;
 
  private:
+  int32_t unique_id_;
+
   int MSAAEvent(ui::AXEvent event);
   bool IsWebAreaForPresentationalIframe();
   bool ShouldNodeHaveReadonlyStateByDefault(const AXNodeData& data) const;