GN: Enable api,media,pc and p2p for the 'webrtc' target.

These parts were commented out to avoid breaking the Chromium
WebRTC FYI bots. Include them in the WebRTC build to make our bots
build as many as possible of our GN targets.

BUG=webrtc:5949
NOTRY=True
TBR=phoglund@webrtc.org

Review-Url: https://codereview.webrtc.org/2054903002
Cr-Original-Commit-Position: refs/heads/master@{#13097}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 94cee3111c19040e16502037b42d2e8a4cae9429
3 files changed