Film grain support adds a huge amount of overhead to the H264Context
structure for a feature that is rarely used. On low end devices or
pages that have lots of media this bloats memory usage rapidly.

This introduces a --disable-h264-film-grain option which makes
these fields optional and reduces the H264Context size from
851808 bytes to 53444 bytes.

Bug: https://crbug.com/359358875
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>

Note: I'm not sure this is the right way to go about making this optional,
please
let me know if there's a better way.

- dale