Don't swap out the old RenderFrameHost until the new one commits.

content/renderer/render_widget.h

Index: content/renderer/render_widget.h
diff --git a/content/renderer/render_widget.h b/content/renderer/render_widget.h
index bd97588d082af4c6fac76ac632063837e651d903..b5ada7d29c1e35536223c437d1b57e89619e1d84 100644
--- a/content/renderer/render_widget.h
+++ b/content/renderer/render_widget.h
@@ -334,6 +334,10 @@ class CONTENT_EXPORT RenderWidget
   // active RenderWidgets.
   void SetSwappedOut(bool is_swapped_out);
 
+  // Allows the process to exit once the unload handler has finished, if there
+  // are no other active RenderWidgets.
+  void WasSwappedOut();
+
   void FlushPendingInputEventAck();
   void DoDeferredClose();
   void DoDeferredSetWindowRect(const blink::WebRect& pos);
@@ -371,7 +375,6 @@ class CONTENT_EXPORT RenderWidget
   virtual void OnWasHidden();
   virtual void OnWasShown(bool needs_repainting,
                           const ui::LatencyInfo& latency_info);
-  virtual void OnWasSwappedOut();
   void OnCreateVideoAck(int32 video_id);
   void OnUpdateVideoAck(int32 video_id);
   void OnRequestMoveAck();