Add new UMA stats to get a handle on Downloads UI Usage

content/browser/download/download_manager_delegate.h

Index: content/browser/download/download_manager_delegate.h
diff --git a/content/browser/download/download_manager_delegate.h b/content/browser/download/download_manager_delegate.h
index 55d25f6a5f469021a3737e40af31f18b346d51fd..1f7c1733a67219af1e71346c01a6194fb5207915 100644
--- a/content/browser/download/download_manager_delegate.h
+++ b/content/browser/download/download_manager_delegate.h
@@ -74,6 +74,8 @@ class DownloadManagerDelegate {
 
   // Notifies the delegate that information about the given download has change,
   // so that it can update its persistent store.
+  // Does not update |url|, |start_time|, |total_bytes|; uses |db_handle| only
+  // to select the row in the database table to update.
   virtual void UpdateItemInPersistentStore(DownloadItem* item) = 0;
 
   // Notifies the delegate that path for the download item has changed, so that