Implement seekable() according to the MediaSource specification

LayoutTests/http/tests/media/media-source/mediasource-util.js

Index: LayoutTests/http/tests/media/media-source/mediasource-util.js
diff --git a/LayoutTests/http/tests/media/media-source/mediasource-util.js b/LayoutTests/http/tests/media/media-source/mediasource-util.js
index e5f567268268e8edf89e2a978b00d29a09264a4a..4b18a74f8554c1185c1dd2357d730e28c0fb14bf 100644
--- a/LayoutTests/http/tests/media/media-source/mediasource-util.js
+++ b/LayoutTests/http/tests/media/media-source/mediasource-util.js
@@ -399,4 +399,10 @@
         assert_equals(actual, expected, description);
     };
 
+    window['assertSeekableEquals'] = function(obj, expected, description)
+    {
+        var actual = timeRangesToString(obj.seekable);
+        assert_equals(actual, expected, description);
+    };
+
 })(window);