Add UMA for studying download cancellation reasons

content/public/browser/android/download_controller_android.h

Index: content/public/browser/android/download_controller_android.h
diff --git a/content/public/browser/android/download_controller_android.h b/content/public/browser/android/download_controller_android.h
index 7eda0e9774b3202944c1e47469fbde90f2ff309b..ce2bb920d3342cf5275ab9c2684a67c933ef2c2f 100644
--- a/content/public/browser/android/download_controller_android.h
+++ b/content/public/browser/android/download_controller_android.h
@@ -25,6 +25,15 @@ class CONTENT_EXPORT DownloadControllerAndroid : public DownloadItem::Observer {
   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,