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/update_attempter.h b/update_attempter.h
index 3e47969..cd7f480 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -86,6 +86,10 @@
   void CheckForUpdate(const std::string& app_version,
                       const std::string& omaha_url);
 
+  // Initiates a reboot if the current state is
+  // UPDATED_NEED_REBOOT. Returns true on sucess, false otherwise.
+  bool RebootIfNeeded();
+
   // DownloadActionDelegate method
   void BytesReceived(uint64_t bytes_received, uint64_t total);