Store NavigationItem's unique ID in navigation context.

ios/web/web_state/navigation_context_impl.mm

Index: ios/web/web_state/navigation_context_impl.mm
diff --git a/ios/web/web_state/navigation_context_impl.mm b/ios/web/web_state/navigation_context_impl.mm
index a1dc6f2dd73942609d48f54bc98e32372fe8fb79..3262e0bd2002337fa6b0f319170dac233228604c 100644
--- a/ios/web/web_state/navigation_context_impl.mm
+++ b/ios/web/web_state/navigation_context_impl.mm
@@ -65,6 +65,14 @@ void NavigationContextImpl::SetResponseHeaders(
   response_headers_ = response_headers;
 }
 
+int NavigationContextImpl::GetNavigationItemUniqueID() const {
+  return navigation_item_unique_id_;
+}
+
+void NavigationContextImpl::SetNavigationItemUniqueID(int unique_id) {
+  navigation_item_unique_id_ = unique_id;
+}
+
 NavigationContextImpl::NavigationContextImpl(
     WebState* web_state,
     const GURL& url,