PlzNavigate: Avoid duplicate SiteInstance creation during navigation.

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 f9e2bf976fcbe85f71d53b78f7fbd8fe5bcf37f9..2c5bd18c595b79f62b3a94a8699613620eeea8c1 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -994,9 +994,20 @@ SiteInstance* RenderFrameHostManager::GetSiteInstanceForNavigation(
       SiteInstanceImpl::GetEffectiveURL(browser_context, dest_url),
       dest_is_view_source_mode);
   if (ShouldTransitionCrossSite() || force_swap) {
-    new_instance = GetSiteInstanceForURL(
-        dest_url, source_instance, current_instance, dest_instance,
-        transition, dest_is_restore, dest_is_view_source_mode, force_swap);
+    if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+            switches::kEnableBrowserSideNavigation) &&
+        speculative_render_frame_host_ &&
+        speculative_render_frame_host_->GetSiteInstance()->GetSiteURL() ==
+            SiteInstanceImpl::GetSiteForURL(browser_context, dest_url)) {

[Charlie Reis, 2015/03/05 04:27:11]

SiteInstanceImpl::GetSiteForURL is not equivalent to
RFHM::GetSiteInstanceForURL, so I think this will have problems.

For example, consider the source_instance case that we use for about:blank and
data: URLs.  GetSiteInstanceForURL puts those in the source SiteInstance, so
their site URL won't match GetSiteForURL(browser_context, "about:blank").

[carlosk, 2015/03/06 16:03:54]

Acknowledged.

+      // We already created a new SiteInstance for this specific URL during this
+      // navigation and used it to create the speculative RenderFrameHost, so
+      // there's no need to create yet another new one.
+      new_instance = speculative_render_frame_host_->GetSiteInstance();
+    } else {
+      new_instance = GetSiteInstanceForURL(
+          dest_url, source_instance, current_instance, dest_instance,
+          transition, dest_is_restore, dest_is_view_source_mode, force_swap);
+    }
   }
 
   // If force_swap is true, we must use a different SiteInstance.  If we didn't,
@@ -1096,7 +1107,7 @@ SiteInstance* RenderFrameHostManager::GetSiteInstanceForURL(
     // create a new SiteInstance.
     if (WebUIControllerFactoryRegistry::GetInstance()->UseWebUIForURL(
             browser_context, dest_url)) {
-        return SiteInstance::CreateForURL(browser_context, dest_url);
+      return SiteInstance::CreateForURL(browser_context, dest_url);
     }
 
     // Normally the "site" on the SiteInstance is set lazily when the load