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

Review-Url: https://codereview.webrtc.org/1979933002
Cr-Original-Commit-Position: refs/heads/master@{#12983}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 164e978f981c7810c4260c4184f41e26bae90230
9 files changed