fix(VTT): Fix timing of VTT in HLS without map header

When there is no X-TIMESTAMP-MAP header, the HLS spec states that the
client should map 0 to 0, effectively.  That means we were wrong to
offset the cues by the segment time.

Since DASH IOP states that segmented text should be packaged in MP4,
this should not affect compliant DASH content.

Closes #2714

Change-Id: Iddb00f2fd1afeb4f0f2c99f92f65e5db0e3a84f1

lib/text/vtt_text_parser.js

@@ -48,7 +48,9 @@ shaka.text.VttTextParser = class {
           shaka.util.Error.Code.INVALID_TEXT_HEADER);
     }
 
-    let offset = time.segmentStart;
+    // NOTE: "periodStart" is the timestamp offset applied via TextEngine.
+    // It is no longer closely tied to periods, but the name stuck around.
+    let offset = time.periodStart;
 
     if (blocks[0].includes('X-TIMESTAMP-MAP')) {
       // https://bit.ly/2K92l7y

test/text/cue_integration.js

@@ -19,7 +19,7 @@ describe('Cue', () => {
         'WEBVTT\n\n' +
         '00:00:20.000 --> 00:00:40.000\n' +
         'Test',
-        {periodStart: 0, segmentStart: 7, segmentEnd: 0});
+        {periodStart: 7, segmentStart: 10, segmentEnd: 60});
     expect(cues.length).toBe(1);
     expect(cues[0].startTime).toBe(27);
     expect(cues[0].endTime).toBe(47);

test/text/vtt_text_parser_unit.js

@@ -83,17 +83,6 @@ describe('VttTextParser', () => {
         {periodStart: 0, segmentStart: 0, segmentEnd: 0});
   });
 
-  it('ignores offset', () => {
-    verifyHelper(
-        [
-          {startTime: 20, endTime: 40, payload: 'Test'},
-        ],
-        'WEBVTT\n\n' +
-        '00:00:20.000 --> 00:00:40.000\n' +
-        'Test',
-        {periodStart: 0, segmentStart: 0, segmentEnd: 0});
-  });
-
   it('supports cues with no settings', () => {
     verifyHelper(
         [
@@ -407,22 +396,22 @@ describe('VttTextParser', () => {
         {periodStart: 0, segmentStart: 0, segmentEnd: 0});
   });
 
-  it('uses segment time', () => {
+  it('uses time offset from periodStart, not segmentStart', () => {
     verifyHelper(
         [
           {
-            startTime: 40, // Note these are 20s off of the cue
-            endTime: 60,   // because using relative timestamps
+            startTime: 70,
+            endTime: 80,
             payload: 'Test',
             textAlign: 'center',
             size: 56,
             writingMode: Cue.writingMode.VERTICAL_LEFT_TO_RIGHT,
           },
         ],
         'WEBVTT\n\n' +
-        '0:00:20.000 --> 0:00:40.000 align:center size:56% vertical:lr\n' +
+        '0:00:10.000 --> 0:00:20.000 align:center size:56% vertical:lr\n' +
         'Test',
-        {periodStart: 0, segmentStart: 20, segmentEnd: 0});
+        {periodStart: 60, segmentStart: 80, segmentEnd: 100});
   });