third_party/libvpx: add BUILD.gn and friends

This CL adds build-related items to our fork of libvpx. The files that weren't
generated are adapted from Chromium, which arranged things a bit differently.
Specifically, the Chromium fork of libvpx is actually at
third_party/libvpx/source/libvpx and the relevant BUILD.gn stuff is in
third_party/libvpx. Our fork doesn't have this nested arrangement.

Files are:
BUILD.gn        - adapted from the Chromium version
README.fuchsia  - adapted from Chromium's README.chromium and modified by
                  generate_gni.sh
generate_gni.sh - generates and updates various files, adapted from the
                  Chromium version
lint_config.sh  - a helper for generate_gni.sh, also adapted from Chromium

All other files added by this CL were generated by generate_gni.sh.

Change-Id: I807584c52d227038db07d93e2d4f0fb93b28d34f
21 files changed