www/chromium: unbreak with ffmpeg 4.0

../../media/ffmpeg/ffmpeg_common.cc:36:1: error: static_assert failed "DecoderBuffer padding size does not fit ffmpeg requirement"
static_assert(DecoderBuffer::kPaddingSize >= AV_INPUT_BUFFER_PADDING_SIZE,
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

PR: 227726
Obtained from: upstream