Mobile API: add root_block_usage_key to response payload.

lms/djangoapps/mobile_api/users/serializers.py

@@ -65,6 +65,7 @@ class CourseOverviewField(serializers.RelatedField):
             "latest_updates": {
                 "video": None
             },
+            "root_block_usage_key": unicode(course_overview.location),
             "video_outline": video_outline_url,
             "course_updates": course_updates_url,
             "course_handouts": course_handouts_url,

lms/djangoapps/mobile_api/users/tests.py

@@ -372,6 +372,7 @@ class TestCourseEnrollmentSerializer(MobileAPITestCase):
         self.assertEqual(serialized['course']['name'], self.course.display_name)
         self.assertEqual(serialized['course']['number'], self.course.id.course)
         self.assertEqual(serialized['course']['org'], self.course.id.org)
+        self.assertEqual(serialized['course']['root_block_usage_key'], unicode(self.course.location))
 
     def test_with_display_overrides(self):
         self.login_and_enroll()