VTTCue: Support 'auto' for line

LayoutTests/media/track/track-add-remove-cue.html

Index: LayoutTests/media/track/track-add-remove-cue.html
diff --git a/LayoutTests/media/track/track-add-remove-cue.html b/LayoutTests/media/track/track-add-remove-cue.html
index 4eb01f120df8e48a960904034d0c3acad3b87401..82e5b7fe63a7aed112a3683327dcff4c5c3eef6d 100644
--- a/LayoutTests/media/track/track-add-remove-cue.html
+++ b/LayoutTests/media/track/track-add-remove-cue.html
@@ -31,7 +31,7 @@
                 testExpected("textCue.pauseOnExit", false);
                 testExpected("textCue.vertical", "");
                 testExpected("textCue.snapToLines", true);
-                testExpected("textCue.line", -1);
+                testExpected("textCue.line", "auto");
                 testExpected("textCue.position", 50);
                 testExpected("textCue.size", 100);
                 testExpected("textCue.align", "middle");
@@ -59,7 +59,7 @@
                 testExpected("newCue.pauseOnExit", false);
                 testExpected("newCue.vertical", "");
                 testExpected("newCue.snapToLines", true);
-                testExpected("newCue.line", -1);
+                testExpected("newCue.line", "auto");
                 testExpected("newCue.position", 50);
                 testExpected("newCue.size", 100);
                 testExpected("newCue.align", "middle");