GN: Add video_engine_tests

Adds separate source_sets for the video_engine_tests subtargets inside
audio, call and video and merges them together into video_engine_tests.

BUG=webrtc:5949
R=kjellander@webrtc.org
TBR=mflodman@webrtc.org

Review URL: https://codereview.webrtc.org/2064523002 .

Cr-Commit-Position: refs/heads/master@{#13127}
diff --git a/webrtc/modules/video_coding/test/receiver_tests.h b/webrtc/modules/video_coding/test/receiver_tests.h
index d6bac07..c2a8477 100644
--- a/webrtc/modules/video_coding/test/receiver_tests.h
+++ b/webrtc/modules/video_coding/test/receiver_tests.h
@@ -29,7 +29,7 @@
 
   int32_t OnReceivedPayloadData(
       const uint8_t* payload_data,
-      const size_t payload_size,
+      size_t payload_size,
       const webrtc::WebRtcRTPHeader* rtp_header) override {
     return vcm_->IncomingPacket(payload_data, payload_size, *rtp_header);
   }