Implement seekable() according to the MediaSource specification

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

 (function(window) {
     // Set the testharness.js timeout to 120 seconds so that it is higher than
     // the LayoutTest timeout. This prevents testharness.js from prematurely
     // terminating tests and allows the LayoutTest runner to control when to
     // timeout the test.
     setup({ explicit_timeout: 120000 });
 
     var SEGMENT_INFO_LIST = [
         {
             url: '/media/resources/media-source/webm/test.webm',
@@ skipping 381 matching lines @@
         }
         return s + " }";
     }
 
     window['assertBufferedEquals'] = function(obj, expected, description)
     {
         var actual = timeRangesToString(obj.buffered);
         assert_equals(actual, expected, description);
     };
 
+    window['assertSeekableEquals'] = function(obj, expected, description)
+    {
+        var actual = timeRangesToString(obj.seekable);
+        assert_equals(actual, expected, description);
+    };
+
 })(window);