Added callback to DownloadUrl() so we can find download failures.

content/public/browser/download_manager.h

Index: content/public/browser/download_manager.h
diff --git a/content/public/browser/download_manager.h b/content/public/browser/download_manager.h
index 12d6fb02b4a121d7ebfae8f2aceca6c00b378279..9c665c7bcc6092921818c94604dac4b191f4b289 100644
--- a/content/public/browser/download_manager.h
+++ b/content/public/browser/download_manager.h
@@ -62,6 +62,8 @@ class WebContents;
 class CONTENT_EXPORT DownloadManager
     : public base::RefCountedThreadSafe<DownloadManager> {
  public:
+  typedef base::Callback<void(DownloadId, net::Error)> OnStartedCallback;
+
   virtual ~DownloadManager() {}
 
   static DownloadManager* Create(
@@ -176,13 +178,16 @@ class CONTENT_EXPORT DownloadManager
   // saved, and whether the user should be prompted about the download.
   // |web_contents| is the web page that the download is done in context of,
   // and must be non-NULL.
+  // |callback| will be called when the download starts, or if an error
+  // occurs.
   virtual void DownloadUrl(const GURL& url,
                            const GURL& referrer,
                            const std::string& referrer_encoding,
                            bool prefer_cache,
                            int64 post_id,
                            const DownloadSaveInfo& save_info,
-                           content::WebContents* web_contents) = 0;
+                           content::WebContents* web_contents,
+                           const OnStartedCallback& callback) = 0;
 
   // Allow objects to observe the download creation process.
   virtual void AddObserver(Observer* observer) = 0;