Change SourceBuffer.appendBuffer() to handle neutered buffers the same way as empty ones.

BUG=367634
TESTS=Test cases added to LayoutTests/http/tests/media/media-source/mediasource-append-buffer.html

Review URL: https://codereview.chromium.org/261223003

git-svn-id: svn://svn.chromium.org/blink/trunk@173340 bbb929c8-8fbe-4397-9dbb-9b2b20218538