Revert "Make rtc_base/refcount.h self contained, not including refcountedobject.h."

This reverts commit b7239a9dc825ddb25dbc800aed3a065163b9a10e.

Reason for revert: Broke chromium mac build, compilation failures on content/renderer/media/webrtc/webrtc_video_frame_adapter.h.

Original change's description:
> Make rtc_base/refcount.h self contained, not including refcountedobject.h.
> 
> The refcount.h file doesn't depend on anything from
> refcountedobject.h. The motivation of this change to make it possible
> to add additional declarations to refcount.h, and include it from
> refcountedobject.h.
> 
> Bug: webrtc:8270
> Change-Id: I24f6131f471e675570968d00065ff9b1f55e3373
> Reviewed-on: https://webrtc-review.googlesource.com/5760
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Commit-Queue: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#20106}

TBR=kwiberg@webrtc.org,nisse@webrtc.org

Change-Id: I7334597cc8979ba9cfaff526967084349ef27f3c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:8270
Reviewed-on: https://webrtc-review.googlesource.com/5800
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20107}

rtc_base/copyonwritebuffer.h

@@ -17,7 +17,6 @@
 #include "rtc_base/buffer.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/refcount.h"
-#include "rtc_base/refcountedobject.h"
 #include "rtc_base/scoped_ref_ptr.h"
 
 namespace rtc {