[Chromecast] Moves persistent UMA client ID handling to common code.

chromecast/browser/metrics/platform_metrics_providers_simple.cc

 // Copyright 2014 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.
 
 #include "chromecast/browser/metrics/platform_metrics_providers.h"
 
 namespace chromecast {
 namespace metrics {
 
 void RegisterPlatformMetricsProviders(
     ::metrics::MetricsService* metrics_service,
     CastService* cast_servce) {
 }
 
-const std::string GetPlatformClientID(CastService* cast_servce) {
-  return "";
-}
-
-::metrics::SystemProfileProto::Channel GetPlatformReleaseChannel(
-     CastService* cast_servce) {
-  return ::metrics::SystemProfileProto::CHANNEL_STABLE;
-}
-
-void PlatformSetClientID(CastService* cast_servce,
-                         const std::string& client_id) {
-}
-
 }  // namespace metrics
 }  // namespace chromecast