Add UMA histogram for the number of SiteInstances per BrowsingInstance.

chrome/browser/site_details.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_SITE_DETAILS_H_
 #define CHROME_BROWSER_SITE_DETAILS_H_
 
 #include "base/containers/hash_tables.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/site_instance.h"
@@ skipping 26 matching lines @@
 // Information about the sites and SiteInstances in each BrowsingInstance, for
 // use in estimating the number of processes needed for various process models.
 struct SiteData {
   SiteData();
   ~SiteData();
 
   // One IsolationScenario object per IsolationScenarioType.
   IsolationScenario scenarios[ISOLATION_SCENARIO_LAST + 1];
 
   // Global list of all SiteInstances, used for de-duping related instances.
-  std::vector<content::SiteInstance*> instances;
+  // It also keeps a set of all SiteInstances in the BrowsingInstance identified
+  // by the SiteInstance used as the key.
+  base::hash_map<content::SiteInstance*, std::set<content::SiteInstance*>>
+      instances;
 
   // A count of all RenderFrameHosts, which are in a different SiteInstance from
   // their parents.
   int out_of_process_frames;
 };
 
 // Maps a BrowserContext to information about the sites it contains.
 typedef base::hash_map<content::BrowserContext*, SiteData>
     BrowserContextSiteDataMap;
 
@@ skipping 11 matching lines @@
 
  private:
   // Never needs to be constructed.
   SiteDetails();
   ~SiteDetails();
 
   DISALLOW_COPY_AND_ASSIGN(SiteDetails);
 };
 
 #endif  // CHROME_BROWSER_SITE_DETAILS_H_