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

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

Index: LayoutTests/http/tests/media/media-source/mediasource-config-changes.js
diff --git a/LayoutTests/http/tests/media/media-source/mediasource-config-changes.js b/LayoutTests/http/tests/media/media-source/mediasource-config-changes.js
index 6fc72782f48949f40f7e45d786bb94482a0d3481..7e8ba725d4857201fa0d6437fcce84cd9d41138a 100644
--- a/LayoutTests/http/tests/media/media-source/mediasource-config-changes.js
+++ b/LayoutTests/http/tests/media/media-source/mediasource-config-changes.js
@@ -64,7 +64,18 @@ function mediaSourceConfigChangeTest(directory, idA, idB, description)
                 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) {