chromeos: Convert D-Bus client classes' callback arguments to const reference

chromeos/dbus/power_manager_client.cc

Index: chromeos/dbus/power_manager_client.cc
diff --git a/chromeos/dbus/power_manager_client.cc b/chromeos/dbus/power_manager_client.cc
index 43532aa9b0c6dbc448e1ad82d24afbd6e8ed40a2..010b45c1e9e16ae0b2ba5edaa7794fe539c1df46 100644
--- a/chromeos/dbus/power_manager_client.cc
+++ b/chromeos/dbus/power_manager_client.cc
@@ -224,7 +224,7 @@ class PowerManagerClientImpl : public PowerManagerClient {
       uint32 request_id,
       uint32 duration,
       int overrides,
-      PowerStateRequestIdCallback callback) OVERRIDE {
+      const PowerStateRequestIdCallback& callback) OVERRIDE {
     dbus::MethodCall method_call(power_manager::kPowerManagerInterface,
                                  power_manager::kStateOverrideRequest);
     dbus::MessageWriter writer(&method_call);
@@ -544,7 +544,7 @@ class PowerManagerClientStubImpl : public PowerManagerClient {
       uint32 request_id,
       uint32 duration,
       int overrides,
-      PowerStateRequestIdCallback callback) OVERRIDE {}
+      const PowerStateRequestIdCallback& callback) OVERRIDE {}
 
   virtual void NotifyScreenLockRequested() OVERRIDE {
     FOR_EACH_OBSERVER(Observer, observers_, LockScreen());