[Alarm] Fix failures raised from test suite.

I've downloaded the test suite from http://otcqa.sh.intel.com/qa-auto/live/Web_TCT_Xwalk/ and run them manually.
All the alarm related failures in the test suite have been fixed.

BUG=XWALK-2163
I had a look at the spec, the `getRemainingSeconds()` should throw an error "if the method cannot be completed".

This test case should be testing for the success case, because the spec says: "If the alarm has expired, this method returns null."