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

transport.h defines an interface for sending rtp and rtcp packets,
which is used by MediaChannel in webrtc/media/engine,
{Audio|Video}{Send|Receive}Stream and in a few other
places. It was part of the build target //webrtc:webrtc, which is a monolithic target with
all webrtc production code. This CL moves the header to its own target in webrtc/api
and deprecates the old location.

Targets in webrtc/api should in general only depend on other
targets in webrtc/api. The target webrtc/api:call_api depends on
transport.h. This change also makes webrtc/voice_engine pass GN's header
include checker and is needed in order for webrtc/api:call_api to pass
it.

transport.h will be completely removed in a follow-up CL in a few weeks
after clients have updated their includes.

NOTRY=True

BUG=webrtc:5589, webrtc:5878, webrtc:6785

Review-Url: https://codereview.webrtc.org/2426563003
Cr-Commit-Position: refs/heads/master@{#15267}
diff --git a/webrtc/video/BUILD.gn b/webrtc/video/BUILD.gn
index 3074d96..aa6730e 100644
--- a/webrtc/video/BUILD.gn
+++ b/webrtc/video/BUILD.gn
@@ -56,6 +56,7 @@
 
   deps = [
     "..:webrtc_common",
+    "../api:transport_api",
     "../base:rtc_analytics",
     "../base:rtc_base_approved",
     "../base:rtc_task_queue",