Tell renderer which subframes have history items on back/forward.

content/browser/frame_host/navigation_entry_impl.cc

Index: content/browser/frame_host/navigation_entry_impl.cc
diff --git a/content/browser/frame_host/navigation_entry_impl.cc b/content/browser/frame_host/navigation_entry_impl.cc
index fbf56542c1609eae1051a8926899c51de2ef4d93..65399b2270b9d1b059d742cf49a5121ef5f08263 100644
--- a/content/browser/frame_host/navigation_entry_impl.cc
+++ b/content/browser/frame_host/navigation_entry_impl.cc
@@ -691,7 +691,7 @@ RequestNavigationParams NavigationEntryImpl::ConstructRequestNavigationParams(
     const FrameNavigationEntry& frame_entry,
     bool is_same_document_history_load,
     bool is_history_navigation_in_new_child,
-    bool has_subtree_history_items,
+    const std::set<std::string>& subframe_unique_names,
     bool has_committed_real_load,
     bool intended_as_new_entry,
     int pending_history_list_offset,
@@ -720,7 +720,7 @@ RequestNavigationParams NavigationEntryImpl::ConstructRequestNavigationParams(
       GetIsOverridingUserAgent(), redirects, GetCanLoadLocalResources(),
       base::Time::Now(), frame_entry.page_state(), GetPageID(), GetUniqueID(),
       is_same_document_history_load, is_history_navigation_in_new_child,
-      has_subtree_history_items, has_committed_real_load, intended_as_new_entry,
+      subframe_unique_names, has_committed_real_load, intended_as_new_entry,
       pending_offset_to_send, current_offset_to_send, current_length_to_send,
       IsViewSourceMode(), should_clear_history_list());
 #if defined(OS_ANDROID)
@@ -847,10 +847,16 @@ FrameNavigationEntry* NavigationEntryImpl::GetFrameEntry(
   return tree_node ? tree_node->frame_entry.get() : nullptr;
 }
 
-bool NavigationEntryImpl::HasSubtreeHistoryItems(
+std::set<std::string> NavigationEntryImpl::GetSubframeUniqueNames(
     FrameTreeNode* frame_tree_node) const {
+  std::set<std::string> names;
   NavigationEntryImpl::TreeNode* tree_node = FindFrameEntry(frame_tree_node);
-  return tree_node && !tree_node->children.empty();
+  if (tree_node) {
+    // Return the names of all immediate children.
+    for (TreeNode* child : tree_node->children)
+      names.insert(child->frame_entry->frame_unique_name());
+  }
+  return names;
 }
 
 void NavigationEntryImpl::ClearStaleFrameEntriesForNewFrame(