Port parts of //third_pary/libjingle build to GN

third_party/libjingle/libjingle.gyp

Index: third_party/libjingle/libjingle.gyp
diff --git a/third_party/libjingle/libjingle.gyp b/third_party/libjingle/libjingle.gyp
index 3e1b8cdf4b2525aea1217b0ecab478bbe68c7ab4..4f3b63c8e1c7a036d71cd758031aac47db35fd8a 100644
--- a/third_party/libjingle/libjingle.gyp
+++ b/third_party/libjingle/libjingle.gyp
@@ -14,6 +14,9 @@
     'libpeer_target_type%': 'static_library',
     'libpeer_allocator_shim%': 0,
   },
+  # Most of these settings have been split according to their scope into
+  # :jingle_unexported_configs, :jingle_direct_dependent_configs,
+  # :jingle_all_dependent_configs in the GN build.
   'target_defaults': {
     'defines': [
       'EXPAT_RELATIVE_PATH',
@@ -48,6 +51,7 @@
       '../../third_party/libyuv/include',
       '../../third_party/usrsctp',
     ],
+    # These dependencies have been translated into :jingle_deps in the GN build.
     'dependencies': [
       '<(DEPTH)/base/base.gyp:base',
       '<(DEPTH)/net/net.gyp:net',
@@ -154,8 +158,8 @@
       'configurations': {
         'Debug': {
           'defines': [
-            # TODO(sergeyu): Fix libjingle to use NDEBUG instead of
-            # _DEBUG and remove this define. See above as well.
+            # TODO(sergeyu): Fix libjingle to use NDEBUG instead of _DEBUG and
+            # remove this define. See above and GN file as well.
             '_DEBUG',
           ],
         }
@@ -260,6 +264,7 @@
     ],
   },
   'targets': [
+    # GN version: //third_party/libjingle
     {
       'target_name': 'libjingle',
       'type': 'static_library',
@@ -282,6 +287,8 @@
     # run GYP and if GYP has removed the validation check, then we can assume
     # that the toolchains have been fixed (we currently use VS2010 and later,
     # so VS2008 isn't a concern anymore).
+    #
+    # GN version: //third_party/libjingle:libjingle_p2p_constants
     {
       'target_name': 'libjingle_p2p_constants',
       'type': 'static_library',
@@ -290,6 +297,7 @@
         '<(libjingle_source)/talk/p2p/base/constants.h',
       ],
     },  # target libjingle_p2p_constants
+    # GN version: //third_party/libjingle:peerconnection_server
     {
       'target_name': 'peerconnection_server',
       'type': 'executable',