Moves VideoSendStreamImpl to a separate file.

This prepares for adding unit tests for VideoSendStreamImpl.

Bug: None
Change-Id: I488041b09f4a455ce4cf1bdc7b8163ef6ad19a8a
Reviewed-on: https://webrtc-review.googlesource.com/70782
Reviewed-by: Benjamin Wright <benwright@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22926}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 309db78..732b83a 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -42,6 +42,8 @@
     "video_receive_stream.h",
     "video_send_stream.cc",
     "video_send_stream.h",
+    "video_send_stream_impl.cc",
+    "video_send_stream_impl.h",
     "video_stream_decoder.cc",
     "video_stream_decoder.h",
     "video_stream_encoder.cc",