Unbundle openh264

This CL makes it easier for Linux distribution packagers to use system
openh264 library, by providing a drop-in replacement GN file.

build/linux/unbundle is a separate directory tree, so that regular
Chromium developers don't need to worry about this configuration.

Please see https://cs.chromium.org/chromium/src/build/linux/unbundle/README?dr
for more context.

BUG=551343
R=jochen@chromium.org, thestig@chromium.org

Review-Url: https://codereview.chromium.org/2888363003 .
Cr-Commit-Position: refs/heads/master@{#473559}
3 files changed