Convert third_party source sets to static libraries.

third_party/libjingle/BUILD.gn

Index: third_party/libjingle/BUILD.gn
diff --git a/third_party/libjingle/BUILD.gn b/third_party/libjingle/BUILD.gn
index eb84f2c2df13a8d5a7ff8ae99db52f9b2e62709d..82db9c7404ea17a4f6bcbc91c2bd033592bb5612 100644
--- a/third_party/libjingle/BUILD.gn
+++ b/third_party/libjingle/BUILD.gn
@@ -232,7 +232,7 @@ if (enable_webrtc) {
   # TODO(kjellander): Move this target into //third_party/webrtc_overrides as
   # soon as the work in bugs.webrtc.org/4256 has gotten rid of the duplicated
   # source listings above.
-  source_set("libjingle_webrtc") {
+  static_library("libjingle_webrtc") {
     sources = [
       "../webrtc_overrides/init_webrtc.cc",
       "../webrtc_overrides/init_webrtc.h",
@@ -244,7 +244,7 @@ if (enable_webrtc) {
     ]
   }
 
-  source_set("libjingle_webrtc_common") {
+  static_library("libjingle_webrtc_common") {
     sources = [
       "../webrtc/api/audiotrack.cc",
       "../webrtc/api/audiotrack.h",
@@ -398,7 +398,7 @@ if (enable_webrtc) {
     ]
   }
 
-  source_set("libstunprober") {
+  static_library("libstunprober") {
     p2p_dir = "../webrtc/p2p"
     sources = [
       "$p2p_dir/stunprober/stunprober.cc",