Add multi_profile_user_count field to system_profile proto.

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: 17
+// Next tag: 18
 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 287 matching lines @@
     optional string name = 1;
     optional string filename = 2;
 
     // The plugin's version.
     optional string version = 3;
 
     // True if the plugin is disabled.
     // If a client has multiple local Chrome user accounts, this is logged based
     // on the first user account launched during the current session.
     optional bool is_disabled = 4;
-    
+
     // True if the plugin is PPAPI.
     optional bool is_pepper = 5;
   }
   repeated Plugin plugin = 7;
 
   // Figures that can be used to generate application stability metrics.
   // All values are counts of events since the last time that these
   // values were reported.
   message Stability {
     // Total amount of time that the program was running, in seconds.
@@ skipping 97 matching lines @@
   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;
+
+  // Number of users currently signed into a multiprofile session.
+  // Non-positive values indicates that the count is unknown or ambiguous.
+  // Logged only on ChromeOS.
+  optional int32 multi_profile_user_count = 17;

[Ilya Sherman, 2013/09/30 23:42:52]

Can this be an unsigned field?  If not, please document what the semantics of
distinct non-positive values are.

[Tim Song, 2013/10/05 01:25:23]

Done. I wanted to leave open the possibility of using negative values to
indicate the number of users when the log is invalidated, but I don't think this
would be really useful. 

 }