Don't split off NTP subframes with TDI.

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 e174e54d379ca834c1016b919b53d5325fb0ce0d..ea32eb5b3caf99da3588bf1330478a5813855194 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -1436,6 +1436,13 @@ RenderFrameHostManager::DetermineSiteInstanceForURL(
       SiteIsolationPolicy::IsTopDocumentIsolationEnabled() &&
       !SiteInstanceImpl::DoesSiteRequireDedicatedProcess(browser_context,
                                                          dest_url)) {
+    if (GetContentClient()
+            ->browser()
+            ->ShouldFrameShareParentSiteInstanceDespiteTopDocumentIsolation(
+                dest_url, current_instance)) {
+      return SiteInstanceDescriptor(render_frame_host_->GetSiteInstance());
+    }
+
     // This is a cross-site subframe of a non-isolated origin, so place this
     // frame in the default subframe site instance.
     return SiteInstanceDescriptor(