Revert 266932 "GN build improvements."

> 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

TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/257243003

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7ae2e5e6e6dbc3e706a0dd2efbe1d923e6406bec