Record Mobile Operator ID in UMA report. This is Android only for now.

chrome/common/metrics/proto/system_profile.proto

 // Copyright (c) 2012 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.
 //
 // Stores information about the user's brower and system configuration.
 // The system configuration fields are recorded once per client session.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
@@ skipping 269 matching lines @@
       WIFI_PHY_LAYER_PROTOCOL_NONE = 0;
       WIFI_PHY_LAYER_PROTOCOL_ANCIENT = 1;
       WIFI_PHY_LAYER_PROTOCOL_A = 2;
       WIFI_PHY_LAYER_PROTOCOL_B = 3;
       WIFI_PHY_LAYER_PROTOCOL_G = 4;
       WIFI_PHY_LAYER_PROTOCOL_N = 5;
       WIFI_PHY_LAYER_PROTOCOL_UNKNOWN = 6;
     }
     // The physical layer mode of the associated wifi access point, if any.
     optional WifiPHYLayerProtocol wifi_phy_layer_protocol = 4;
+
+    // MCC+MNC (mobile country code + mobile network code) of the
+    // the current registered mobile network operator.
+    // If there is no mobile network, the value is the empty string.

[Alexei Svitkine (slow), 2014/04/23 17:58:49]

How about when it changes during the the course of an UMA log?

I think in that case it would be best to also report an empty string. This would
require a bit more involved changes to the metrics code - e.g. keeping a
timestamp of when it changed and compare that to the start of the log. If you
look at the code for synthetic trials, they do something similar.

[bolian, 2014/04/24 00:59:24]

Is MetricsLog::RecordEnvironment eventually called when I call, for example,
UMA_HISTOGRAM_COUNTS, so that the environment is recorded for that particular
metric?

Or, MetricsLog::RecordEnvironment is actually called when reporting the metrics
through WiFi?

If it is the first case, I think the chance to record a metric during network
transition is low, and we can accept some errors. If it is the latter case, I
have to reconsider.

+    optional string mcc_mnc = 5;
   }
   optional Network network = 13;
 
   // Information on the Google Update install that is managing this client.
   message GoogleUpdate {
     // Whether the Google Update install is system-level or user-level.
     optional bool is_system_install = 1;
 
     // The date at which Google Update last started performing an automatic
     // update check, in seconds since the Unix epoch.
@@ skipping 200 matching lines @@
   // client_id:
   //   client_key = client_id % 4096
   // Then, each installed extension is mapped into a hash bucket according to
   //   bucket = CityHash64(StringPrintf("%d:%s",
   //                                    client_key, extension_id)) % 1024
   // The client reports the set of hash buckets occupied by all installed
   // extensions.  If multiple extensions map to the same bucket, that bucket is
   // still only reported once.
   repeated int32 occupied_extension_bucket = 18;
 }