Add the Sync.Attachments.StoreInitResult histogram.

sync/api/attachments/attachment_store.h

 // Copyright 2014 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 SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_H_
 #define SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_H_
 
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 15 matching lines @@
 
 // A place to locally store and access Attachments.
 //
 // Destroying this object does not necessarily cancel outstanding async
 // operations. If you need cancel like semantics, use WeakPtr in the callbacks.
 class SYNC_EXPORT AttachmentStoreBase {
  public:
   // TODO(maniscalco): Consider udpating Read and Write methods to support
   // resumable transfers (bug 353292).
 
+  // The result status of an attachment store operation.
+  // Do not re-order or delete these entries; they are used in a UMA histogram.
   enum Result {
-    SUCCESS,            // No error, all completed successfully.
-    UNSPECIFIED_ERROR,  // An unspecified error occurred for one or more items.
-    STORE_INITIALIZATION_FAILED,  // AttachmentStore initialization failed.
+    SUCCESS = 0,            // No error, all completed successfully.
+    UNSPECIFIED_ERROR = 1,  // An unspecified error occurred for >= 1 item.
+    STORE_INITIALIZATION_FAILED = 2,  // AttachmentStore initialization failed.
+    // When adding a value here, you must increment RESULT_SIZE below.
   };
+  const int RESULT_SIZE = 10; // Size of the Result enum; used for histograms.
 
   typedef base::Callback<void(const Result&)> InitCallback;
   typedef base::Callback<void(const Result&,
                               scoped_ptr<AttachmentMap>,
                               scoped_ptr<AttachmentIdList>)> ReadCallback;
   typedef base::Callback<void(const Result&)> WriteCallback;
   typedef base::Callback<void(const Result&)> DropCallback;
   typedef base::Callback<void(const Result&,
                               scoped_ptr<AttachmentMetadataList>)>
       ReadMetadataCallback;
@@ skipping 89 matching lines @@
       const InitCallback& callback);
 
  protected:
   friend class base::RefCountedThreadSafe<AttachmentStore>;
   ~AttachmentStore() override;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_H_