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

content/test/web_contents_observer_sanity_checker.cc

Index: content/test/web_contents_observer_sanity_checker.cc
diff --git a/content/test/web_contents_observer_sanity_checker.cc b/content/test/web_contents_observer_sanity_checker.cc
index a27c96b3ac16ba636529829552bea3589e59eec2..ce22bedd7038e8cec3a2594d0d70cc6b0bcc48bd 100644
--- a/content/test/web_contents_observer_sanity_checker.cc
+++ b/content/test/web_contents_observer_sanity_checker.cc
@@ -54,6 +54,12 @@ void WebContentsObserverSanityChecker::RenderFrameCreated(
       static_cast<RenderFrameHostImpl*>(render_frame_host)->IsRenderFrameLive())
       << "RenderFrameCreated called on for a RenderFrameHost that thinks it is "
          "not alive.";
+
+  // Any child frame must be in the same BrowsingInstance as its parent.
+  if (render_frame_host->GetParent()) {
+    CHECK(render_frame_host->GetSiteInstance()->IsRelatedSiteInstance(
+        render_frame_host->GetParent()->GetSiteInstance()));
+  }
 }
 
 void WebContentsObserverSanityChecker::RenderFrameDeleted(