Implement InstanceID API functions.

components/gcm_driver/instance_id/instance_id_driver.cc

Index: components/gcm_driver/instance_id/instance_id_driver.cc
diff --git a/components/gcm_driver/instance_id/instance_id_driver.cc b/components/gcm_driver/instance_id/instance_id_driver.cc
index f0ea456a65f4e86eaeca57aa2b14d92059c69e31..edb5270ff0bd2868bc1b28425522f5c48c4ef4c2 100644
--- a/components/gcm_driver/instance_id/instance_id_driver.cc
+++ b/components/gcm_driver/instance_id/instance_id_driver.cc
@@ -4,10 +4,29 @@
 
 #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_) {