[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