Make GCMDriver::GetInstanceIDHandler protected, to avoid misuse

components/gcm_driver/instance_id/instance_id.cc

Index: components/gcm_driver/instance_id/instance_id.cc
diff --git a/components/gcm_driver/instance_id/instance_id.cc b/components/gcm_driver/instance_id/instance_id.cc
index 9e875a02ab4a6852f20f4557e54f3a4a06535183..4d933398e53c2ae84bdfff0c2b9f0d5f7b0c734a 100644
--- a/components/gcm_driver/instance_id/instance_id.cc
+++ b/components/gcm_driver/instance_id/instance_id.cc
@@ -4,10 +4,13 @@
 
 #include "components/gcm_driver/instance_id/instance_id.h"
 
+#include "components/gcm_driver/gcm_driver.h"
+
 namespace instance_id {
 
-InstanceID::InstanceID(const std::string& app_id)
-    : app_id_(app_id) {
+InstanceID::InstanceID(const std::string& app_id, gcm::GCMDriver* gcm_driver)
+    : handler_(gcm_driver->GetInstanceIDHandlerInternal()), app_id_(app_id) {
+  DCHECK(handler_);
 }
 
 InstanceID::~InstanceID() {