Check updating attribute when set timestampOffset for SourceBuffer.

Error should be thrown when try to set timestampOffset for pending sourceBuffer.

BUG=239506
TEST=Updated http/tests/media/media-source/mediasource-append-buffer.html LayoutTest.

Review URL: https://chromiumcodereview.appspot.com/21207002

git-svn-id: svn://svn.chromium.org/blink/trunk@155537 bbb929c8-8fbe-4397-9dbb-9b2b20218538