Send a subtree of same-process PageStates for back/forward child frames.

content/renderer/render_view_impl.h

Index: content/renderer/render_view_impl.h
diff --git a/content/renderer/render_view_impl.h b/content/renderer/render_view_impl.h
index 09d45bc64d71e7b0d670a9f08cb5066be0041350..ffa350e5f15413845c47abe9a28416a628eca33c 100644
--- a/content/renderer/render_view_impl.h
+++ b/content/renderer/render_view_impl.h
@@ -717,6 +717,23 @@ class CONTENT_EXPORT RenderViewImpl
   // are gone.
   int frames_in_progress_;
 
+  // Keeps track of the unique names of future subframes that the browser
+  // process has history items for during a history navigation, as well as the
+  // corresponding PageState to use when creating such frames during a
+  // back/forward navigation.  This map includes state for any frames in the
+  // page, even when concurrent back/forward navigations affect different
+  // subtrees.
+  //
+  // If the PageState for a frame is empty, the history item needs to load in a
+  // different process, so the renderer process should ask the browser process
+  // to handle it instead
+  //
+  // This state is incrementally cleared as it is used.
+  // TODO(creis): Can it be reset at any point?  This seems important, so that a
+  // frame on one page doesn't use an unused history item from the previous
+  // page.
+  std::unordered_map<std::string, PageState> history_page_states_;
+
   // UI state ------------------------------------------------------------------
 
   // The state of our target_url transmissions. When we receive a request to