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

.gn

Index: .gn
diff --git a/.gn b/.gn
index 7a98fadc3657f619e766ab1e254d736f3a2c13de..d19be4d8431dba936d4b7908dff641d95c0840c9 100644
--- a/.gn
+++ b/.gn
@@ -21,6 +21,7 @@ secondary_source = "//build/secondary/"
 # TODO(kjellander): Keep adding paths to this list as work in webrtc:5589 is done.
 check_targets = [
   "//webrtc/api:rtc_stats_api",
+  "//webrtc/api:transport_api",
   "//webrtc/modules/audio_coding:g711_test",
   "//webrtc/modules/audio_coding:g722_test",
   "//webrtc/modules/audio_coding:ilbc_test",
@@ -33,6 +34,7 @@ check_targets = [
   "//webrtc/modules/audio_device/*",
   "//webrtc/modules/audio_mixer/*",
   "//webrtc/stats:rtc_stats",
+  "//webrtc/voice_engine",
   "//webrtc/voice_engine:level_indicator",
 ]