Reason for revert:
Too many errors to address showed up when trying to land this with Chromium changes in https://codereview.chromium.org/2022833002/.
Will address them separately before relanding.
Original issue's description:
> Reland of GN: Add BUILD.gn files for webrtc/{api,media,libjingle,p2p,pc}
>
> Add BUILD.gn files for webrtc/{api,media,libjingle,p2p,pc} in
> preparation for removing src/third_party/libjingle in Chromium.
>
> Changes from previous attempt:
> * Added libstunprober target
> * Adjusted warnings for Chromium's clang plugins
> * webrtc/pc/externalhmac.{h,cc} added for Chromium builds.
>
> As soon this has landed a roll including the changes in
> https://codereview.chromium.org/2022833002/ is needed to make
> Chromium build cleanly.
>
> BUG=webrtc:4256
> NOTRY=True
> NOPRESUBMIT=True
>
> Committed: https://crrev.com/164e978f981c7810c4260c4184f41e26bae90230
> Cr-Commit-Position: refs/heads/master@{#12983}
TBR=perkj@webrtc.org,tommi@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:4256
Review-Url: https://codereview.webrtc.org/2023233002
Cr-Commit-Position: refs/heads/master@{#12988}
4d167e5ccd