Change UMA proto product field to be an int32.

components/metrics/proto/chrome_user_metrics_extension.proto

 // 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.
 //
 // Protocol buffer for Chrome UMA (User Metrics Analysis).
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package metrics;
 
 import "histogram_event.proto";
 import "omnibox_event.proto";
 import "profiler_event.proto";
 import "system_profile.proto";
 import "user_action_event.proto";
 import "perf_data.proto";
 import "sampled_profile.proto";
 
 // Next tag: 12
 message ChromeUserMetricsExtension {
   // The product (i.e. end user application) for a given UMA log.
   enum Product {
     // Google Chrome product family.
     CHROME = 0;
   }
-  // The product corresponding to this log. Note: The default value is Chrome,
-  // so Chrome products will not transmit this field.
-  optional Product product = 10 [default = CHROME];
+  // The product corresponding to this log. The field type is int32 instead of
+  // Product so that downstream users of the Chromium metrics component can
+  // introduce products without needing to make changes to the Chromium code
+  // (though they still need to add the new product to the server-side enum).
+  // Note: The default value is Chrome, so Chrome products will not transmit
+  // this field.
+  optional int32 product = 10 [default = 0];
 
   // The id of the client install that generated these events.
   //
   // For Chrome clients, this id is unique to a top-level (one level above the
   // "Default" directory) Chrome user data directory [1], and so is shared among
   // all Chrome user profiles contained in this user data directory.
   // An id of 0 is reserved for test data (monitoring and internal testing) and
   // should normally be ignored in analysis of the data.
   // [1] http://www.chromium.org/user-experience/user-data-directory
   optional fixed64 client_id = 1;
@@ skipping 13 matching lines @@
   repeated OmniboxEventProto omnibox_event = 5;
   repeated HistogramEventProto histogram_event = 6;
   repeated ProfilerEventProto profiler_event = 7;
 
   // TODO(sque): Deprecate this field and use |sampled_profile| instead.
   repeated PerfDataProto perf_data = 8;
 
   // A list of all collected sample-based profiles since the last UMA upload.
   repeated SampledProfile sampled_profile = 11;
 }