[third_party/ffmpeg] add files for building with Chromium repo

This CL adds files to third_party/ffmpeg that are used to build
ffmpeg for fuchsia from an unaltered import of Chromium's ffmpeg,
which is imported as third_party/ffmpeg/src.

The following have been added:

config/               generated source files for specific configs
scripts/              scripts used during the roll process
BUILD.gn              builds ffmpeg from src/ and config/
ffmpeg_generated.gni  generated file used in the build
ffmpeg_options.gni    file used in the build
OWNERS
README.fuchsia        roll instructions, etc
LICENSE.md            license file
CREDITS.fuchsia       generated credits

This change will break the prebuilder until the Chromium ffmpeg repo
is integrated at third_party/ffmpeg/src.

Test: build and roll processes manually tested

Change-Id: I372d24f95691a03c4fd07ba06d70dce811a44a7a
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/ffmpeg/+/736703
Reviewed-by: Adam Barth <abarth@google.com>
60 files changed