[Sync] Add comment that ClientToServerMessage.share can be empty.

sync/protocol/sync.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.
 //
 // Sync protocol for communication between sync client and server.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 712 matching lines @@
 message DataTypeContext {
   // The type this context is associated with.
   optional int32 data_type_id = 1;
   // The context for the datatype.
   optional bytes context = 2;
   // The version of the context.
   optional int64 version = 3;
 }
 
 message ClientToServerMessage {
+  // |share| field is only used on the server for logging and can sometimes
+  // contain empty string. It is still useful for logging username when it can't
+  // be derived from access token in case of auth error.
   required string share = 1;
+
   optional int32 protocol_version = 2 [default = 45];
   enum Contents {
     COMMIT = 1;
     GET_UPDATES = 2;
     AUTHENTICATE = 3;
     DEPRECATED_4 = 4;
     CLEAR_SERVER_DATA = 5;
   }
 
   // Each ClientToServerMessage contains one request defined by the
@@ skipping 275 matching lines @@
 };
 
 // A message indicating that the sync engine has been disabled on a client.
 message SyncDisabledEvent {
   // The GUID that identifies the sync client.
   optional string cache_guid = 1;
 
   // The store birthday that the client was using before disabling sync.
   optional string store_birthday = 2;
 };