Enable building WebRTC with GN.

This CL depends on the following CL to be landed first:
https://webrtc-codereview.appspot.com/15989004
with the addition that that WebRTC revision of it needs to be
be rolled into Chromium DEPS first.

BUG=webrtc:3441
TEST=See https://webrtc-codereview.appspot.com/15989004 for details.

Review URL: https://codereview.chromium.org/321313006

Cr-Commit-Position: refs/heads/master@{#291688}

BUILD.gn

@@ -107,6 +107,7 @@ group("root") {
     "//third_party/snappy",
     "//third_party/speex",
     "//third_party/widevine/cdm:version_h",
+    "//third_party/webrtc",
     "//third_party/zlib",
     "//third_party:jpeg",
     "//tools/gn",