Consolidate all TDI SiteInstances into one render process.

content/browser/site_instance_impl.h

Index: content/browser/site_instance_impl.h
diff --git a/content/browser/site_instance_impl.h b/content/browser/site_instance_impl.h
index 3b05af89bf16585b084d402b36fe60c7d1f1c578..fc2ad85bd30738cf2167cc2bd8735aeebe7ced99 100644
--- a/content/browser/site_instance_impl.h
+++ b/content/browser/site_instance_impl.h
@@ -144,6 +144,11 @@ class CONTENT_EXPORT SiteInstanceImpl final : public SiteInstance,
   // Used to restrict a process' origin access rights.
   void LockToOrigin();
 
+  // This gets the render process to use for default subframe site instances.
+  RenderProcessHost* GetDefaultSubframeProcessHost(
+      BrowserContext* browser_context,
+      bool is_for_guests_only);
+
   void set_is_default_subframe_site_instance() {
     is_default_subframe_site_instance_ = true;
   }