GN build improvements.

Fixes a TODO about private symbols on GCC.

Moves many third party BUILD.gn files from the secondary tree to the main one.

Fixes the dependencies on Linux so "gn check" passes again.

BUG=367595
R=scottmg@chromium.org

Previous review: https://codereview.chromium.org/256153003/

Review URL: https://codereview.chromium.org/264463002

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 71d94dfce9c1989595d60e2737e05420cf6a5d5e