Ensure window reference is valid when navigating cross-process and back.

content/browser/frame_host/render_frame_host_manager.cc

Index: content/browser/frame_host/render_frame_host_manager.cc
diff --git a/content/browser/frame_host/render_frame_host_manager.cc b/content/browser/frame_host/render_frame_host_manager.cc
index 1ffbbaa4861f5795c19410c982d11b26da767cdd..79a477d1e020d3b3b20b25c1fb7d121c842a1110 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -912,10 +912,13 @@ void RenderFrameHostManager::DiscardUnusedFrame(
 
 void RenderFrameHostManager::MoveToPendingDeleteHosts(
     scoped_ptr<RenderFrameHostImpl> render_frame_host) {
-  // If this is the main frame going away, mark the corresponding
-  // RenderViewHost for deletion as well so that we don't try to reuse it.
-  if (render_frame_host->frame_tree_node()->IsMainFrame())
+  // If this is the main frame going away and there are no more references to
+  // its RenderViewHost, mark it for deletion as well so that we don't try to
+  // reuse it.
+  if (render_frame_host->frame_tree_node()->IsMainFrame() &&
+      render_frame_host->render_view_host()->ref_count() <= 1) {
     render_frame_host->render_view_host()->set_pending_deletion();
+  }
 
   // |render_frame_host| will be deleted when its SwapOut ACK is received, or
   // when the timer times out, or when the RFHM itself is deleted (whichever