Add UMA histogram for the number of SiteInstances per BrowsingInstance.

chrome/browser/site_details.h

Index: chrome/browser/site_details.h
diff --git a/chrome/browser/site_details.h b/chrome/browser/site_details.h
index a81fc379ea059aac5b0df18eff577a46d33ab613..507b7cbd41682dd0f86e6797e99653462bd09b59 100644
--- a/chrome/browser/site_details.h
+++ b/chrome/browser/site_details.h
@@ -44,7 +44,10 @@ struct SiteData {
   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.