Always create FrameTreeNodes and RenderFrameHosts for every frame.

content/browser/frame_host/frame_tree_node.cc

Index: content/browser/frame_host/frame_tree_node.cc
diff --git a/content/browser/frame_host/frame_tree_node.cc b/content/browser/frame_host/frame_tree_node.cc
index ce357750443542d1ea3f4f1380313d6196b111e1..0d77618a729efc248dacd4799b72c7fb1cfc524e 100644
--- a/content/browser/frame_host/frame_tree_node.cc
+++ b/content/browser/frame_host/frame_tree_node.cc
@@ -56,11 +56,13 @@ void FrameTreeNode::RemoveChild(FrameTreeNode* child) {
 
 void FrameTreeNode::ResetForMainFrame(
     RenderFrameHostImpl* new_render_frame_host) {
-  DCHECK_EQ(0UL, children_.size());
-
   owns_render_frame_host_ = false;
   frame_id_ = kInvalidFrameId;
   current_url_ = GURL();
+
+  // The children may not have been cleared if a cross-process navigation
+  // commits before the old process cleans everything up.  Make sure the child
+  // nodes get deleted.
   children_.clear();
 
   render_frame_host_ = new_render_frame_host;