Make the Get Alarm Callback Parameter Optional

In debug builds, the callback parameters are validated. Since this value
is not sent on an alarm that doesn't exist, the validation fails.

The fix here is to make the parameter optional.

BUG=369636

Review URL: https://codereview.chromium.org/267723014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267974 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/api/alarms.idl

@@ -40,7 +40,7 @@ namespace alarms {
     double? periodInMinutes;
   };
 
-  callback AlarmCallback = void (Alarm alarm);
+  callback AlarmCallback = void (optional Alarm alarm);
   callback AlarmListCallback = void (Alarm[] alarms);
   callback ClearCallback = void (boolean wasCleared);