Always swap with a replacement proxy in OnSwapOut.

content/browser/frame_host/frame_tree.h

Index: content/browser/frame_host/frame_tree.h
diff --git a/content/browser/frame_host/frame_tree.h b/content/browser/frame_host/frame_tree.h
index fe57d89eadc60c22988db211e318882378e8e589..7993316e1ccc18f9024bd5e5811ca81b5444498d 100644
--- a/content/browser/frame_host/frame_tree.h
+++ b/content/browser/frame_host/frame_tree.h
@@ -211,7 +211,6 @@ class CONTENT_EXPORT FrameTree {
   friend class FrameTreeTest;
   FRIEND_TEST_ALL_PREFIXES(RenderFrameHostImplBrowserTest, RemoveFocusedFrame);
   typedef base::hash_map<int, RenderViewHostImpl*> RenderViewHostMap;
-  typedef std::multimap<int, RenderViewHostImpl*> RenderViewHostMultiMap;
 
   // Returns a range to iterate over all FrameTreeNodes in the frame tree in
   // breadth-first traversal order, skipping the subtree rooted at
@@ -235,13 +234,6 @@ class CONTENT_EXPORT FrameTree {
   // the map will be cleared before we delete the RenderFrameHosts in the tree.
   RenderViewHostMap render_view_host_map_;
 
-  // Map of SiteInstance ID to RenderViewHosts that are pending shutdown. The
-  // renderers of these RVH are currently executing the unload event in
-  // background. When the SwapOutACK is received, they will be deleted. In the
-  // meantime, they are kept in this map, as they should not be reused (part of
-  // their state is already gone away).
-  RenderViewHostMultiMap render_view_host_pending_shutdown_map_;
-
   // This is an owned ptr to the root FrameTreeNode, which never changes over
   // the lifetime of the FrameTree. It is not a scoped_ptr because we need the
   // pointer to remain valid even while the FrameTreeNode is being destroyed,