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..8ffd6abdeace2bab1bde29f543505c297bc4e2fe 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -1269,6 +1269,12 @@ bool RenderFrameHostManager::ShouldSwapBrowsingInstancesForNavigation(
     SiteInstance* new_site_instance,
     const GURL& new_effective_url,
     bool new_is_view_source_mode) const {
+  // A subframe must stay in the same BrowsingInstance as its parent.
+  // TODO(nasko): Ensure that SiteInstance swap is still triggered for subframes
+  // in the cases covered by the rest of the checks in this method.
+  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) {