Implement the renderer side of the mojo based local storage implementation.

content/renderer/dom_storage/local_storage_area.h

Index: content/renderer/dom_storage/local_storage_area.h
diff --git a/content/renderer/dom_storage/local_storage_area.h b/content/renderer/dom_storage/local_storage_area.h
index 1da7af016e34358bcb14d505d53b49b4cd1b0fe2..312521545139d6f9d6fff1ce207aa78bbddfd4a1 100644
--- a/content/renderer/dom_storage/local_storage_area.h
+++ b/content/renderer/dom_storage/local_storage_area.h
@@ -32,8 +32,13 @@ class LocalStorageArea : public blink::WebStorageArea {
                   const blink::WebURL& page_url) override;
   void clear(const blink::WebURL& url) override;
 
+  const std::string& id() const { return id_; }
+
  private:
   scoped_refptr<LocalStorageCachedArea> cached_area_;
+  // A globally unique identifier for this storage area. It's used to pass the
+  // source storage area, if any, in mutation events.
+  std::string id_;
 
   DISALLOW_COPY_AND_ASSIGN(LocalStorageArea);
 };