Inside of content, prefer SiteInstanceImpl to SiteInstance.

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 9089a02ab89c4a7ec8983b2cd35fd22dca5ba034..8b25838828942a642265f463e67de531dcf05105 100644
--- a/content/browser/site_instance_impl.h
+++ b/content/browser/site_instance_impl.h
@@ -50,6 +50,9 @@ class CONTENT_EXPORT SiteInstanceImpl final : public SiteInstance,
   size_t GetRelatedActiveContentsCount() override;
   bool RequiresDedicatedProcess() override;
 
+  // Same as GetRelatedSiteInstance, but returns a SiteInstanceImpl.
+  scoped_refptr<SiteInstanceImpl> GetRelatedSiteInstanceImpl(const GURL& url);
+
   // Returns the SiteInstance, related to this one, that should be used
   // for subframes when an oopif is required, but a dedicated process is not.
   // This SiteInstance will be created if it doesn't already exist. There is