PlzNavigate: Avoid duplicate SiteInstance creation during navigation.

content/browser/frame_host/render_frame_host_manager.h

Index: content/browser/frame_host/render_frame_host_manager.h
diff --git a/content/browser/frame_host/render_frame_host_manager.h b/content/browser/frame_host/render_frame_host_manager.h
index 15338be2d0de70f6d680a0daf3b0359aa460b58d..6ce5c1b394f79ba9a1e5eb501b4e9ebf8fee06a6 100644
--- a/content/browser/frame_host/render_frame_host_manager.h
+++ b/content/browser/frame_host/render_frame_host_manager.h
@@ -488,6 +488,14 @@ class CONTENT_EXPORT RenderFrameHostManager : public NotificationObserver {
                                       bool dest_is_view_source_mode,
                                       bool force_browsing_instance_swap);
 
+  // Creates a new SiteInstance for the provided URL under a new browsing
+  // instance.
+  // PlzNavigate: If a SiteInstance was already previously created for this URL
+  // for the same navigation, return that one. Otherwise create a new one and
+  // return it.
+  SiteInstance* CreateSiteInstanceForURL(BrowserContext* browser_context,
+                                         const GURL& dest_url);
+
   // Determines the appropriate url to use as the current url for SiteInstance
   // selection.
   const GURL& GetCurrentURLForSiteInstance(