AU: Provide a reboot_if_needed D-Bus API.

This will reboot if the current state is UPDATED_REBOOT_NEEDED.
Also add -reboot option to the update_engine_client.

BUG=4250
TEST=unit tests,gmerged on device and tried reboot requests

Review URL: http://codereview.chromium.org/3034026
diff --git a/UpdateEngine.conf b/UpdateEngine.conf
index 05f0056..94aa5a5 100644
--- a/UpdateEngine.conf
+++ b/UpdateEngine.conf
@@ -22,6 +22,9 @@
     <allow send_destination="org.chromium.UpdateEngine"
            send_interface="org.chromium.UpdateEngineInterface"
            send_member="GetStatus"/>
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
+           send_member="RebootIfNeeded"/>
   </policy>
   <policy context="default">
     <deny send_destination="org.chromium.UpdateEngine" />