Collect some histograms about signed binary downloads.

chrome/browser/safe_browsing/safe_browsing_service.h

Index: chrome/browser/safe_browsing/safe_browsing_service.h
===================================================================
--- chrome/browser/safe_browsing/safe_browsing_service.h	(revision 106761)
+++ chrome/browser/safe_browsing/safe_browsing_service.h	(working copy)
@@ -267,6 +267,8 @@
     return csd_service_.get();
   }
 
+  // The DownloadProtectionService is not valid after the SafeBrowsingService
+  // is destroyed.
   safe_browsing::DownloadProtectionService*
       download_protection_service() const {
     return download_service_.get();
@@ -472,6 +474,10 @@
   // starts or stops the service accordingly.
   void RefreshState();
 
+  // Arranges to delete |download_service_| on the IO thread, and sets
+  // the pointer to NULL.
+  void ShutDownDownloadService();
+
   // The factory used to instanciate a SafeBrowsingService object.
   // Useful for tests, so they can provide their own implementation of
   // SafeBrowsingService.
@@ -556,7 +562,7 @@
 
   // The DownloadProtectionService is managed by the SafeBrowsingService,
   // since its running state and lifecycle depends on SafeBrowsingService's.
-  scoped_refptr<safe_browsing::DownloadProtectionService> download_service_;
+  scoped_ptr<safe_browsing::DownloadProtectionService> download_service_;
 
   DISALLOW_COPY_AND_ASSIGN(SafeBrowsingService);
 };