Making rtc_base:ptr_util and rtc_base:refcount public.

Bug: None
Change-Id: I6f4b372c087c6d25e8c451ab5577cb3bcb13f6f0
Reviewed-on: https://webrtc-review.googlesource.com/93284
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24237}

rtc_base/BUILD.gn

@@ -110,12 +110,14 @@ rtc_source_set("platform_thread_types") {
 }
 
 rtc_source_set("ptr_util") {
+  visibility = [ "*" ]
   sources = [
     "scoped_ref_ptr.h",
   ]
 }
 
 rtc_source_set("refcount") {
+  visibility = [ "*" ]
   sources = [
     "refcount.h",
     "refcountedobject.h",