sync: Add DeviceInfo protobuf and supporting code

sync/protocol/device_info_specifics.proto

Index: sync/protocol/device_info_specifics.proto
diff --git a/sync/protocol/device_info_specifics.proto b/sync/protocol/device_info_specifics.proto
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+// 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 datatype extension for nigori keys.
+
+// Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
+// any fields in this file.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+option retain_unknown_fields = true;
+
+package sync_pb;
+
+import "sync_enums.proto";
+
+// Information about a device that is running a sync-enabled Chrome browser.
+// We are mapping the unique per-device cache guid to more specific information
+// about the device.
+message DeviceInfoSpecifics {
+  // The unique cache_guid created to identify a sync client on this device.
+  optional string cache_guid = 1;
+
+  // A non-unique but human readable name to describe this client.
+  optional string client_name = 2;
+
+  // The platform of the device (ChromeOS, Linux, Mac, or Windows).

[Nicolas Zea, 2012/09/26 01:50:59]

remove the examples of platforms, it's better to just let someone look at the
enum def.

[rlarocque, 2012/09/26 20:44:36]

Done.

+  optional SyncEnums.DeviceType device_type = 3;
+
+  // The UserAgent used when contacting the Chrome Sync server.
+  optional string sync_user_agent = 4;

[Nicolas Zea, 2012/09/26 01:50:59]

I don't think it makes sense to sync this around. We had the version previously
for the sake of dashboard's consumption (I believe). The sync user agent is in
my view a union of various fields, which if we need we should set explicitly.

[rlarocque, 2012/09/26 20:44:36]

Let's bring this up with Raz.  We should also double-check to see what the
dashboard's requirements are.

+}