Move RefCountInterface to api/ and webrtc: namespace

This CL just moves the definition and adds a forward.
Actually using the new definition is left for later CLs.

Bug: webrtc:15622
Change-Id: I6d97ef45b98f9eb193c59dd7f8a89c99cfe0ba9a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/325381
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41048}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index 46703f93..1eb3876 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -438,6 +438,11 @@
   absl_deps = [ "//third_party/abseil-cpp/absl/memory" ]
 }
 
+rtc_source_set("ref_count") {
+  visibility = [ "*" ]
+  sources = [ "ref_count.h" ]
+}
+
 rtc_source_set("scoped_refptr") {
   visibility = [ "*" ]
   sources = [ "scoped_refptr.h" ]