MSE: Add layout test for 'decode' error via an algorithm

LayoutTests/http/tests/media/media-source/mediasource-append-buffer.html

Index: LayoutTests/http/tests/media/media-source/mediasource-append-buffer.html
diff --git a/LayoutTests/http/tests/media/media-source/mediasource-append-buffer.html b/LayoutTests/http/tests/media/media-source/mediasource-append-buffer.html
index 16b0d919cb52ed726bd7b33258ba07d5f31080b7..ce6044796f26c2f5ac54c0e8e7d9c655fd95ff53 100644
--- a/LayoutTests/http/tests/media/media-source/mediasource-append-buffer.html
+++ b/LayoutTests/http/tests/media/media-source/mediasource-append-buffer.html
@@ -324,7 +324,7 @@
           mediasource_testafterdataloaded(function(test, mediaElement, mediaSource, segmentInfo, sourceBuffer, mediaData)
           {
               var initSegment = MediaSourceUtil.extractSegmentData(mediaData, segmentInfo.init);
-              var halfIndex = (initSegment.length + 1) / 2;
+              var halfIndex = (initSegment.length - 1) / 2;
               var partialInitSegment = initSegment.subarray(0, halfIndex);
               var remainingInitSegment = initSegment.subarray(halfIndex);
               var mediaSegment = MediaSourceUtil.extractSegmentData(mediaData, segmentInfo.media[0]);
@@ -363,7 +363,7 @@
           {
               var initSegment = MediaSourceUtil.extractSegmentData(mediaData, segmentInfo.init);
               var mediaSegment = MediaSourceUtil.extractSegmentData(mediaData, segmentInfo.media[0]);
-              var halfIndex = (mediaSegment.length + 1) / 2;
+              var halfIndex = (mediaSegment.length - 1) / 2;
               var partialMediaSegment = mediaSegment.subarray(0, halfIndex);
               var remainingMediaSegment = mediaSegment.subarray(halfIndex);