For both alignment cue setting value and position cue setting alignment value,

allow "center".
This value (and not "middle") is listed in
https://w3c.github.io/webvtt/ ( WebVTT: The Web Video Text Tracks Format, Draft
Community Group Report, 21 December 2015).

Leaving the behavior for "middle" unchanged.
It was the value listed in older drafts, e.g.
https://www.w3.org/TR/2014/WD-webvtt1-20141113/ ( W3C First Public Working
Draft 13 November 2014 )
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117220612

library/src/androidTest/assets/webvtt/with_positioning

@@ -26,3 +26,8 @@ NOTE The positioning alignment should be inherited from align.
 00:07.000 --> 00:08.000 position:10% align:end size:10%
 This is the fifth subtitle.
 
+NOTE In newer drafts, align:middle has been replaced by align:center
+
+00:10.000 --> 00:11.000 line:45%,end align:center size:35%
+This is the sixth subtitle.
+

library/src/androidTest/java/com/google/android/exoplayer/text/webvtt/WebvttParserTest.java

@@ -109,7 +109,7 @@ public class WebvttParserTest extends InstrumentationTestCase {
     WebvttSubtitle subtitle = parser.parse(bytes, 0, bytes.length);
 
     // test event count
-    assertEquals(10, subtitle.getEventTimeCount());
+    assertEquals(12, subtitle.getEventTimeCount());
 
     // test cues
     assertCue(subtitle, 0, 0, 1234000, "This is the first subtitle.", Alignment.ALIGN_NORMAL,
@@ -126,6 +126,9 @@ public class WebvttParserTest extends InstrumentationTestCase {
     assertCue(subtitle, 8, 7000000, 8000000, "This is the fifth subtitle.",
         Alignment.ALIGN_OPPOSITE, Cue.DIMEN_UNSET, Cue.TYPE_UNSET, Cue.TYPE_UNSET, 0.1f,
         Cue.ANCHOR_TYPE_END, 0.1f);
+  assertCue(subtitle, 10, 10000000, 11000000, "This is the sixth subtitle.",
+        Alignment.ALIGN_CENTER, 0.45f, Cue.LINE_TYPE_FRACTION, Cue.ANCHOR_TYPE_END, Cue.DIMEN_UNSET,
+        Cue.TYPE_UNSET, 0.35f);
   }
 
   public void testParseWithBadCueHeader() throws IOException {

library/src/main/java/com/google/android/exoplayer/text/webvtt/WebvttCueParser.java

@@ -275,6 +275,7 @@ public final class WebvttCueParser {
     switch (s) {
       case "start":
         return Cue.ANCHOR_TYPE_START;
+      case "center":
       case "middle":
         return Cue.ANCHOR_TYPE_MIDDLE;
       case "end":
@@ -290,6 +291,7 @@ public final class WebvttCueParser {
       case "start":
       case "left":
         return Alignment.ALIGN_NORMAL;
+      case "center":
       case "middle":
         return Alignment.ALIGN_CENTER;
       case "end":