Send Omaha event_type 54 after reboot to new update.

When rebooting to a new version, we were sending an Omaha event_type 3
to flag a "success reboot". Nevertheless, the server expect us to send
only one event_type 3 for every <updatecheck> response with a payload
but we were sending one event_type 3 upon payload application and
another one after reboot.

This patch changes the event_type sent after reboot to 54, a value
reserved from Chromium OS clients.

Bug: None
Test: Added unittest.

Change-Id: I199a2b00c702175762f2c154ca2f45b3f6ad768c
diff --git a/omaha_request_action.h b/omaha_request_action.h
index f1b81ec..57ef250 100644
--- a/omaha_request_action.h
+++ b/omaha_request_action.h
@@ -59,13 +59,15 @@
     kTypeUpdateComplete = 3,
     kTypeUpdateDownloadStarted = 13,
     kTypeUpdateDownloadFinished = 14,
+    // Chromium OS reserved type sent after the first reboot following an update
+    // completed.
+    kTypeRebootedAfterUpdate = 54,
   };
 
   // The Result values correspond to EVENT_RESULT values of Omaha.
   enum Result {
     kResultError = 0,
     kResultSuccess = 1,
-    kResultSuccessReboot = 2,
     kResultUpdateDeferred = 9,  // When we ignore/defer updates due to policy.
   };