Rewrite DownloadsApiTest in C++.

chrome/common/chrome_notification_types.h

Index: chrome/common/chrome_notification_types.h
diff --git a/chrome/common/chrome_notification_types.h b/chrome/common/chrome_notification_types.h
index bf61354122576cf005b50756d4c4164051a961e8..071c061f855001d29c639d1e655a3e49ed1a2dd4 100644
--- a/chrome/common/chrome_notification_types.h
+++ b/chrome/common/chrome_notification_types.h
@@ -577,6 +577,11 @@ enum NotificationType {
   // details are a pointer to the const BookmarksFunction in question.
   NOTIFICATION_EXTENSION_BOOKMARKS_API_INVOKED,
 
+  // Sent when a downloads extensions API event is fired. The source is an
+  // ExtensionDownloadsEventRouter::NotificationSource, and the details is a
+  // std::string containing json. Used for testing.
+  NOTIFICATION_EXTENSION_DOWNLOADS_EVENT,
+
   // Sent when an omnibox extension has sent back omnibox suggestions. The
   // source is the profile, and the details are an ExtensionOmniboxSuggestions
   // object.