PlzNavigate: add cancel navigation logic for uncommitted requests

content/browser/frame_host/navigation_request.h

Index: content/browser/frame_host/navigation_request.h
diff --git a/content/browser/frame_host/navigation_request.h b/content/browser/frame_host/navigation_request.h
index d8be70ba1bee21c7df45b73a99e291d60ee8f863..85873be27722a2c32046bc59a1cbd1129706d8a0 100644
--- a/content/browser/frame_host/navigation_request.h
+++ b/content/browser/frame_host/navigation_request.h
@@ -31,7 +31,13 @@ class NavigationRequest {
   // manipulated on the UI thread.
   void BeginNavigation(scoped_refptr<ResourceRequestBody> body);
 
+  int64 navigation_request_id() const { return navigation_request_id_; }
+
  private:
+  // The next available browser-global navigation request ID.
+  static int64 next_navigation_request_id_;
+
+  const int64 navigation_request_id_;
   const NavigationRequestInfo info_;
   const int64 frame_node_id_;