Add skeleton webrtc::SessionDescription and webrtc::MediaDescription classes.

webrtc/api/BUILD.gn

Index: webrtc/api/BUILD.gn
diff --git a/webrtc/api/BUILD.gn b/webrtc/api/BUILD.gn
index e42b834c5c86d01bdddbb5b0df67673fa32e4b45..71e5243f90336e3f614fd1bf42f739a52ac304d2 100644
--- a/webrtc/api/BUILD.gn
+++ b/webrtc/api/BUILD.gn
@@ -88,12 +88,16 @@ rtc_static_library("libjingle_peerconnection_api") {
 rtc_source_set("ortc_api") {
   check_includes = false  # TODO(deadbeef): Remove (bugs.webrtc.org/6828)
   sources = [
+    "ortc/mediadescription.cc",
+    "ortc/mediadescription.h",
     "ortc/ortcfactoryinterface.h",
     "ortc/ortcrtpreceiverinterface.h",
     "ortc/ortcrtpsenderinterface.h",
     "ortc/packettransportinterface.h",
     "ortc/rtptransportcontrollerinterface.h",
     "ortc/rtptransportinterface.h",
+    "ortc/sessiondescription.cc",
+    "ortc/sessiondescription.h",
     "ortc/srtptransportinterface.h",
     "ortc/udptransportinterface.h",
   ]
@@ -105,6 +109,10 @@ rtc_source_set("ortc_api") {
   public_deps = [
     ":libjingle_peerconnection_api",
   ]
+  if (!build_with_chromium && is_clang) {
+    # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
+    suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
+  }
 }
 
 # TODO(ossu): Remove once downstream projects have updated.
@@ -223,6 +231,8 @@ if (rtc_include_tests) {
   rtc_source_set("rtc_api_unittests") {
     testonly = true
     sources = [
+      "ortc/mediadescription_unittest.cc",
+      "ortc/sessiondescription_unittest.cc",
       "rtcerror_unittest.cc",
     ]
 
@@ -233,6 +243,7 @@ if (rtc_include_tests) {
 
     deps = [
       ":libjingle_peerconnection_api",
+      ":ortc_api",
       "//webrtc/test:test_support",
     ]
   }