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

Review URL: https://codereview.chromium.org/256153003

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ff0811a8763c69ee59e3e6d0421f2f43ced7d41a