Reland of GN: Add BUILD.gn files for webrtc/{api,media,libjingle,p2p,pc}

webrtc/BUILD.gn

Index: webrtc/BUILD.gn
diff --git a/webrtc/BUILD.gn b/webrtc/BUILD.gn
index e1aeb45720577402d1692d2c1f6cc20db23309b7..374e6526425713a48b328846045993f09d9cdb14 100644
--- a/webrtc/BUILD.gn
+++ b/webrtc/BUILD.gn
@@ -84,6 +84,10 @@ config("common_config") {
     all_dependent_configs = [ "dbus-glib" ]
   }
 
+  if (rtc_relative_path) {
+    defines += [ "EXPAT_RELATIVE_PATH" ]
+  }
+
   if (build_with_chromium) {
     defines += [ "LOGGING_INSIDE_WEBRTC" ]
   } else {
@@ -182,11 +186,13 @@ source_set("webrtc") {
 
   deps = [
     ":webrtc_common",
+    "api",
     "audio",
     "base:rtc_base",
     "call",
     "common_audio",
     "common_video",
+    "media",
     "modules/audio_coding",
     "modules/audio_conference_mixer",
     "modules/audio_device",
@@ -198,6 +204,8 @@ source_set("webrtc") {
     "modules/utility",
     "modules/video_coding",
     "modules/video_processing",
+    "p2p",
+    "pc",
     "system_wrappers",
     "tools",
     "video",