Add the Sync.Attachments.StoreInitResult histogram.

sync/api/attachments/attachment_store.h

Index: sync/api/attachments/attachment_store.h
diff --git a/sync/api/attachments/attachment_store.h b/sync/api/attachments/attachment_store.h
index d6d0d3953b34115e2463bae23a93b1172d106bdc..a3c823d8adc3e804273c6c51624c446108c5fd4d 100644
--- a/sync/api/attachments/attachment_store.h
+++ b/sync/api/attachments/attachment_store.h
@@ -33,11 +33,15 @@ class SYNC_EXPORT AttachmentStoreBase {
   // 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.
+    // When adding a value here, you must increment RESULT_SIZE below.
   };
+  const int RESULT_SIZE = 3; // Size of the Result enum; used for histograms.

[pavely, 2014/12/10 22:48:34]

Ok, let's assign explicit values to constants so that reordering and deletion
doesn't affect histogram and set RESULT_SIZE to 10 so that it covers future
possible values.

 
   typedef base::Callback<void(const Result&)> InitCallback;
   typedef base::Callback<void(const Result&,