Implement InstanceID API functions.

components/gcm_driver/instance_id/instance_id_driver.cc

 // Copyright 2015 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 "components/gcm_driver/instance_id/instance_id_driver.h"
 
+#include "base/metrics/field_trial.h"
 #include "components/gcm_driver/instance_id/instance_id.h"
 
 namespace instance_id {
 
+namespace {
+#if !defined(OS_ANDROID)
+const char kInstanceIDFieldTrialName[] = "InstanceID";
+const char kInstanceIDFieldTrialEnabledGroupName[] = "Enabled";
+#endif    // !defined(OS_ANDROID)
+}  // namespace
+
+// static
+bool InstanceIDDriver::IsInstanceIDEnabled() {
+#if defined(OS_ANDROID)
+  return true;
+#else
+  std::string group_name =
+      base::FieldTrialList::FindFullName(kInstanceIDFieldTrialName);
+  return group_name == kInstanceIDFieldTrialEnabledGroupName;
+#endif    // defined(OS_ANDROID)
+}
+
 InstanceIDDriver::InstanceIDDriver(gcm::GCMDriver* gcm_driver)
     : gcm_driver_(gcm_driver),
       instance_id_map_deleter_(&instance_id_map_) {
 }
 
 InstanceIDDriver::~InstanceIDDriver() {
 }
 
 InstanceID* InstanceIDDriver::GetInstanceID(const std::string& app_id) {
   auto iter = instance_id_map_.find(app_id);
   if (iter != instance_id_map_.end())
     return iter->second;
 
   InstanceID* instance_id = InstanceID::Create(app_id, gcm_driver_);
   instance_id_map_[app_id] = instance_id;
   return instance_id;
 }
 
 }  // namespace instance_id