Moved transport.h from webrtc/ to webrtc/api, created build target and updated dependencies.

webrtc/api/BUILD.gn

Index: webrtc/api/BUILD.gn
diff --git a/webrtc/api/BUILD.gn b/webrtc/api/BUILD.gn
index 01aacb745928c2f1ed09e47a6e4dd577084b31ff..fc392b2184938cac6c370c5f162fe3da1b82cc70 100644
--- a/webrtc/api/BUILD.gn
+++ b/webrtc/api/BUILD.gn
@@ -37,6 +37,7 @@ rtc_source_set("call_api") {
   deps = [
     # TODO(kjellander): Add remaining dependencies when webrtc:4243 is done.
     ":audio_mixer_api",
+    ":transport_api",
     "..:webrtc_common",
     "../base:rtc_base_approved",
     "../modules/audio_coding:audio_encoder_interface",
@@ -341,6 +342,11 @@ rtc_source_set("audio_mixer_api") {
   ]
 }
 
+rtc_source_set("transport_api") {
+  sources = [
+    "call/transport.h",
+  ]
+}
 if (rtc_include_tests) {
   config("peerconnection_unittests_config") {
     # The warnings below are enabled by default. Since GN orders compiler flags