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..d89c997efe3d91cd39b861df3ce75a2eda2cacaa 100644
--- a/components/gcm_driver/instance_id/instance_id.cc
+++ b/components/gcm_driver/instance_id/instance_id.cc
@@ -6,8 +6,10 @@
 
 namespace instance_id {
 
-InstanceID::InstanceID(const std::string& app_id)
-    : app_id_(app_id) {
+InstanceID::InstanceID(const std::string& app_id,
+                       gcm::InstanceIDHandler* handler)
+    : handler_(handler), app_id_(app_id) {
+  DCHECK(handler_);
 }
 
 InstanceID::~InstanceID() {