Upload UMA data using protocol buffers.

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;
 
 package metrics;
 
-// Next tag: 9
+// Next tag: 10
 message SystemProfileProto {
   // The time when the client was compiled/linked, in seconds since the epoch.
   optional int64 build_timestamp = 1;
 
   // A version number string for the application.
   // Most commonly this is the browser version number found in a user agent
   // string, and is typically a 4-tuple of numbers separated by periods.  In
   // cases where the user agent version might be ambiguous (example: Linux 64-
   // bit build, rather than 32-bit build, or a Windows version used in some
   // special context, such as ChromeFrame running in IE), then this may include
@@ skipping 194 matching lines @@
       // increase this count multiple times.)
       optional int32 instance_count = 3;
 
       // The number of times this plugin process crashed.
       // This value will be <= |launch_count|.
       optional int32 crash_count = 4;
     }
     repeated PluginStability plugin_stability = 22;
   }
   optional Stability stability = 8;
+
+  // Description of a field trial or experiment that the user is currently
+  // enrolled in.
+  // All metrics reported in this upload can potentially be influenced by the
+  // field trial.
+  message FieldTrial {
+    // The name of the field trial, as a 32-bit identifier.
+    // Currently, the identifier is a hash of the field trial's name.
+    optional fixed32 name_id = 1;
+
+    // The user's group within the field trial, as a 32-bit identifier.
+    // Currently, the identifier is a hash of the group's name.
+    optional fixed32 group_id = 2;
+  }
+  repeated FieldTrial field_trial = 9;
 }