UMA stats for MTP file system in Chrome OS Files.app.

chrome/browser/chromeos/file_manager/volume_manager.h

Index: chrome/browser/chromeos/file_manager/volume_manager.h
diff --git a/chrome/browser/chromeos/file_manager/volume_manager.h b/chrome/browser/chromeos/file_manager/volume_manager.h
index 492d387da0726d0acbce839e0900ce85f5901450..3cfc2b102f566f4d610fa272c88ac47599634a1f 100644
--- a/chrome/browser/chromeos/file_manager/volume_manager.h
+++ b/chrome/browser/chromeos/file_manager/volume_manager.h
@@ -40,8 +40,9 @@ namespace file_manager {
 class MountedDiskMonitor;
 class VolumeManagerObserver;
 
-// This manager manages "Drive" and "Downloads" in addition to disks managed
-// by DiskMountManager.
+// Identifiers for volume types managed by Chrome OS file manager.
+// The list must be kept in sync with FileManagerVolumeType enum
+// in tools/metrics/histograms/histograms.xml for recording user metric.
 enum VolumeType {
   VOLUME_TYPE_GOOGLE_DRIVE,
   VOLUME_TYPE_DOWNLOADS_DIRECTORY,
@@ -50,7 +51,8 @@ enum VolumeType {
   VOLUME_TYPE_CLOUD_DEVICE,
   VOLUME_TYPE_PROVIDED,  // File system provided by the FileSystemProvider API.
   VOLUME_TYPE_MTP,
-  VOLUME_TYPE_TESTING
+  VOLUME_TYPE_TESTING,  // Used only in tests.

[Ilya Sherman, 2014/05/08 21:16:11]

nit: Since enums used for histograms should be append-only, I'd recommend making
this be the first item in the enum rather than the last.  That way, you don't
end up with gaps in terms of which items are relevant for histograms.xml.

(I'd also add a quick comment to document that this enum should be treated as
append-only.)

[kinaba, 2014/05/09 02:09:27]

Thanks for the suggestion. Done.

+  NUM_VOLUME_TYPE,
 };
 
 struct VolumeInfo {