Prevent creating a swapped out RVH in the same SiteInstance as the current one.

content/browser/frame_host/render_view_host_manager.h

Index: content/browser/frame_host/render_view_host_manager.h
diff --git a/content/browser/frame_host/render_view_host_manager.h b/content/browser/frame_host/render_view_host_manager.h
index 377758f882843a67510a74187f12c1a07910826e..01a0cdadb59bf09a58e38cdf7f384a2080744d64 100644
--- a/content/browser/frame_host/render_view_host_manager.h
+++ b/content/browser/frame_host/render_view_host_manager.h
@@ -308,11 +308,12 @@ class CONTENT_EXPORT RenderViewHostManager
       const NavigationEntryImpl* new_entry) const;
 
   // Returns an appropriate SiteInstance object for the given NavigationEntry,
-  // possibly reusing the current SiteInstance.
-  // Never called if --process-per-tab is used.
+  // possibly reusing the current SiteInstance.  If --process-per-tab is used,
+  // this is only called when ShouldSwapProcessesForNavigation reutrns true.
   SiteInstance* GetSiteInstanceForEntry(
       const NavigationEntryImpl& entry,
-      SiteInstance* curr_instance);
+      SiteInstance* curr_instance,
+      bool force_swap);
 
   // Sets up the necessary state for a new RenderViewHost with the given opener.
   bool InitRenderView(RenderViewHost* render_view_host, int opener_route_id);