Add UMA for studying download cancellation reasons

content/public/browser/android/download_controller_android.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_PUBLIC_BROWSER_ANDROID_DOWNLOAD_CONTROLLER_ANDROID_H_
 #define CONTENT_PUBLIC_BROWSER_ANDROID_DOWNLOAD_CONTROLLER_ANDROID_H_
 
 #include "base/callback.h"
 #include "content/common/content_export.h"
 #include "content/public/browser/download_item.h"
 #include "content/public/common/context_menu_params.h"
 
 namespace content {
 class DownloadItem;
 class WebContents;
 
 // Interface to request GET downloads and send notifications for POST
 // downloads.
 class CONTENT_EXPORT DownloadControllerAndroid : public DownloadItem::Observer {
  public:
   // Returns the singleton instance of the DownloadControllerAndroid.
   static DownloadControllerAndroid* Get();
 
   // Called to set the DownloadControllerAndroid instance.
   static void SetDownloadControllerAndroid(
       DownloadControllerAndroid* download_controller);
 
+  enum DownloadCancelReason {

[Ilya Sherman, 2016/06/10 21:20:43]

nit: Please document that this enum is used to back an UMA histogram, and
therefore should be treated as append-only.

[qinmin, 2016/06/11 00:13:00]

Done.

+    kActionButton = 0,
+    kNotificationDismissed,
+    kOverwriteInfobarDismissed,
+    kNoStoragePermission, // new enums must be added after this.
+    kCancelReasonMax

[Ilya Sherman, 2016/06/10 21:20:43]

nit: Also, please either define this as an enum class, or use a common prefix
for each constant name, like CANCEL_REASON_.

[Ilya Sherman, 2016/06/10 21:20:43]

nit: Please use SHOUTY_CASE for enum names, per
https://www.chromium.org/developers/coding-style#TOC-Naming

[qinmin, 2016/06/11 00:13:00]

Done.

[qinmin, 2016/06/11 00:13:00]

Done.

+  };
+  static void RecordDownloadCancelReason(DownloadCancelReason reason);
+
   // Starts a new download request with Android. Should be called on the
   // UI thread.
   virtual void CreateGETDownload(int render_process_id, int render_view_id,
                                  int request_id, bool must_download) = 0;
 
   // Should be called when a download is started. It can be either a GET
   // request with authentication or a POST request. Notifies the embedding
   // app about the download. Should be called on the UI thread.
   virtual void OnDownloadStarted(DownloadItem* download_item) = 0;
 
@@ skipping 25 matching lines @@
   virtual void SetDefaultDownloadFileName(const std::string& file_name) {}
 
  protected:
   ~DownloadControllerAndroid() override {};
   static DownloadControllerAndroid* download_controller_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_ANDROID_DOWNLOAD_CONTROLLER_ANDROID_H_