PlzNavigate: Speculatively spawns a renderer process for navigations.

content/browser/site_instance_impl.cc

Index: content/browser/site_instance_impl.cc
diff --git a/content/browser/site_instance_impl.cc b/content/browser/site_instance_impl.cc
index f1555dc1af8f627c7eb50d811a3ae445adec9678..d9069548b379896267977d569ec1a89bb73b5013 100644
--- a/content/browser/site_instance_impl.cc
+++ b/content/browser/site_instance_impl.cc
@@ -141,6 +141,7 @@ void SiteInstanceImpl::SetSite(const GURL& url) {
   // be fixed, but until then, we should still not set the site of a
   // SiteInstance more than once.
   DCHECK(!has_site_);
+  DCHECK(!browsing_instance_->HasSiteInstance(url));
 
   // Remember that this SiteInstance has been used to load a URL, even if the
   // URL is invalid.
@@ -181,6 +182,10 @@ SiteInstance* SiteInstanceImpl::GetRelatedSiteInstance(const GURL& url) {
   return browsing_instance_->GetSiteInstanceForURL(url);
 }
 
+SiteInstance* SiteInstanceImpl::GetRelatedEmptySiteInstance() {
+  return new SiteInstanceImpl(browsing_instance_.get());
+}
+
 bool SiteInstanceImpl::IsRelatedSiteInstance(const SiteInstance* instance) {
   return browsing_instance_.get() == static_cast<const SiteInstanceImpl*>(
                                          instance)->browsing_instance_.get();