Enforce all subframes to be in the same BrowsingInstance as the parent.

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 67a150e11a6ac9700f05fa5aa63cc57d0bd0075a..491a01a2827eccaecc4dd17ecf0ee4517fe4213a 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -1269,6 +1269,10 @@ bool RenderFrameHostManager::ShouldSwapBrowsingInstancesForNavigation(
     SiteInstance* new_site_instance,
     const GURL& new_effective_url,
     bool new_is_view_source_mode) const {
+  // Subframe must stay in the same BrowsingInstance as its parent.

[Charlie Reis, 2015/12/04 18:23:23]

nit: A subframe

Do we want a TODO for "Ensure that we still trigger SiteInstance swaps in
subframes for the cases checked below"?

[nasko, 2015/12/04 18:32:33]

Done.

+  if (!frame_tree_node_->IsMainFrame())
+    return false;
+
   // If new_entry already has a SiteInstance, assume it is correct.  We only
   // need to force a swap if it is in a different BrowsingInstance.
   if (new_site_instance) {