Fix doubling of offset for segment-relative cues

When using segment-relative timestamps in VTT, the presentation
offset has already been factored into the segment time.  Therefore,
it should not be added again in the VTT parser.

Closes #595
Closes #599

Change-Id: I9d062af7a17859f6f3374ecf20369b361f3eac7b

Author: joeyparrish

lib/media/vtt_text_parser.js

@@ -112,13 +112,13 @@ shaka.media.VttTextParser.parseCue_ =
         shaka.util.Error.Code.INVALID_TEXT_CUE);
   }
 
-  start += offset;
-  end += offset;
-
   // See issue #480 for discussion on deprecation
   if (useRelativeCueTimestamps) {
     start += segmentStartTime;
     end += segmentStartTime;
+  } else {
+    start += offset;
+    end += offset;
   }
 
   // Get the payload.

test/media/vtt_text_parser_unit.js

@@ -356,7 +356,7 @@ describe('VttTextParser', function() {
         'WEBVTT\n\n' +
         '0:00:20.000 --> 0:00:40.000 align:center size:56% vertical:lr\n' +
         'Test',
-        undefined,
+        /* offset */ 7,  // ignored when using relative timestamps
         20,
         true);
   });