Move videosourceinterface to api.

VideoSourceInterface is clearly an integral part of
mediastreaminterface.h already, so moving that interface to api makes
sense. This also resolves a circular dependency in call/.

Bug: webrc:6828
Change-Id: Ic1862f118363b0b55a235a9c0c35d9adc647184c
Reviewed-on: https://webrtc-review.googlesource.com/37500
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21498}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index 05c0aee..8bbfc26 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -73,6 +73,8 @@
     "umametrics.cc",
     "umametrics.h",
     "videosinkinterface.h",
+    "videosourceinterface.cc",
+    "videosourceinterface.h",
     "videosourceproxy.h",
   ]