Drop include of refcountedobject.h.

This is a followup to cl https://webrtc-review.googlesource.com/7281, and
a reland of the difficult part of cl
https://webrtc-review.googlesource.com/5760.

Bug: webrtc:8270
Change-Id: I92eb47083b47038015496e797a9ec18f75e607ca
Reviewed-on: https://webrtc-review.googlesource.com/7607
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20270}

rtc_base/refcount.h

diff --git a/rtc_base/refcount.h b/rtc_base/refcount.h
index 6b49dec..f29d279 100644
--- a/rtc_base/refcount.h
+++ b/rtc_base/refcount.h
@@ -10,10 +10,6 @@
 #ifndef RTC_BASE_REFCOUNT_H_
 #define RTC_BASE_REFCOUNT_H_
 
-// TODO(nisse): Delete this include. We plan to let refcountedobject.h
-// include this file, rather than the other way around.
-#include "rtc_base/refcountedobject.h"
-
 namespace rtc {
 
 // Reference count interface.