Fix MediaSource.duration setter behavior to match the current spec.

LayoutTests/http/tests/media/media-source/mediasource-config-changes.js

 // Extract & return the resolution string from a filename, if any.
 function resolutionFromFilename(filename)
 {
     resolution = filename.replace(/^.*[^0-9]([0-9]+x[0-9]+)[^0-9].*$/, "$1");
     if (resolution != filename) {
         return resolution;
     }
     return "";
 }
 
@@ skipping 46 matching lines @@
                 {
                     // Add the second buffer starting at 1.5 second.
                     sourceBuffer.timestampOffset = 1.5;
                     appendBuffer(test, sourceBuffer, dataB);
                     ++expectedResizeEventCount;
                 });
 
                 test.waitForExpectedEvents(function()
                 {
                     // Truncate the presentation to a duration of 2 seconds.
+                    assert_false(sourceBuffer.updating, "sourceBuffer.updating");
+
                     mediaSource.duration = 2;
+
+                    assert_true(sourceBuffer.updating, "sourceBuffer.updating");
+                    test.expectEvent(sourceBuffer, "updatestart");
+                    test.expectEvent(sourceBuffer, "update");
+                    test.expectEvent(sourceBuffer, "updateend");
+                });
+
+                test.waitForExpectedEvents(function()
+                {
                     mediaSource.endOfStream();
 
                     if (expectResizeEvents) {
                         for (var i = 0; i < expectedResizeEventCount; ++i) {
                             test.expectEvent(mediaElement, "resize");
                         }
                     }
                     test.expectEvent(mediaElement, "ended");
                     mediaElement.play();
                 });
 
                 test.waitForExpectedEvents(function() {
                     test.done();
                 });
             });
         });
     }, description, { timeout: 10000 } );
 };