Moved transport_adapter.h/.cc from call/ to video/ dir to remove circular dependency

Issue: video_receive_stream.cc includes transport_adapter.h which use to be inside call/ and call depends on video/ which caused circular dependency. We moved transport_adapter.h/.cc inside video/ and removed dependency of video/ on call/

BUG=webrtc:6412
NOTRY=True

Review-Url: https://codereview.webrtc.org/2470913004
Cr-Commit-Position: refs/heads/master@{#14907}
diff --git a/webrtc/video/BUILD.gn b/webrtc/video/BUILD.gn
index aa42dc0..e97de6f 100644
--- a/webrtc/video/BUILD.gn
+++ b/webrtc/video/BUILD.gn
@@ -34,6 +34,8 @@
     "stats_counter.h",
     "stream_synchronization.cc",
     "stream_synchronization.h",
+    "transport_adapter.cc",
+    "transport_adapter.h",
     "video_decoder.cc",
     "video_encoder.cc",
     "video_receive_stream.cc",