Add histogram for number of out-of-process iframes.

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 da613373d276a0103dcfbc20212c03f54523d1b3..db779fa16b5339c90a0b52d3c267aea06efa7745 100644
--- a/chrome/browser/site_details.h
+++ b/chrome/browser/site_details.h
@@ -45,6 +45,10 @@ struct SiteData {
 
   // Global list of all SiteInstances, used for de-duping related instances.
   std::vector<content::SiteInstance*> instances;
+
+  // A set of all RenderFrameHosts, which are in a different SiteInstance from
+  // their parents.
+  std::set<content::RenderFrameHost*> out_of_process_frames;
 };
 
 // Maps a BrowserContext to information about the sites it contains.