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

This is a reland of b7239a9dc825ddb25dbc800aed3a065163b9a10e
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}

Bug: webrtc:8270
Change-Id: I9738f6680ab52d0f43639a1a39175fdba5957681
Reviewed-on: https://webrtc-review.googlesource.com/5840
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20180}

rtc_base/bind_unittest.cc

@@ -14,6 +14,7 @@
 #include "rtc_base/gunit.h"
 
 #include "rtc_base/refcount.h"
+#include "rtc_base/refcountedobject.h"
 
 namespace rtc {
 

rtc_base/callback.h

@@ -13,7 +13,7 @@
  */
 
 // To generate callback.h from callback.h.pump, execute:
-// /home/build/google3/third_party/gtest/scripts/pump.py callback.h.pump
+// ../third_party/googletest/src/googletest/scripts/pump.py callback.h.pump
 
 // Callbacks are callable object containers. They can hold a function pointer
 // or a function object and behave like a value type. Internally, data is
@@ -63,6 +63,7 @@
 #define RTC_BASE_CALLBACK_H_
 
 #include "rtc_base/refcount.h"
+#include "rtc_base/refcountedobject.h"
 #include "rtc_base/scoped_ref_ptr.h"
 
 namespace rtc {

rtc_base/callback.h.pump

@@ -9,7 +9,7 @@
  */
 
 // To generate callback.h from callback.h.pump, execute:
-// /home/build/google3/third_party/gtest/scripts/pump.py callback.h.pump
+// ../third_party/googletest/src/googletest/scripts/pump.py callback.h.pump
 
 // Callbacks are callable object containers. They can hold a function pointer
 // or a function object and behave like a value type. Internally, data is
@@ -58,6 +58,7 @@
 #define RTC_BASE_CALLBACK_H_
 
 #include "rtc_base/refcount.h"
+#include "rtc_base/refcountedobject.h"
 #include "rtc_base/scoped_ref_ptr.h"
 
 namespace rtc {

rtc_base/copyonwritebuffer.h

@@ -17,6 +17,7 @@
 #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 {

rtc_base/refcount.h

@@ -10,8 +10,6 @@
 #ifndef RTC_BASE_REFCOUNT_H_
 #define RTC_BASE_REFCOUNT_H_
 
-#include "rtc_base/refcountedobject.h"
-
 namespace rtc {
 
 // Reference count interface.

rtc_base/refcountedobject_unittest.cc

@@ -12,6 +12,7 @@
 
 #include "rtc_base/gunit.h"
 #include "rtc_base/refcount.h"
+#include "rtc_base/refcountedobject.h"
 
 namespace rtc {
 

rtc_base/rtccertificate.cc

@@ -13,6 +13,7 @@
 #include "rtc_base/rtccertificate.h"
 
 #include "rtc_base/checks.h"
+#include "rtc_base/refcountedobject.h"
 
 namespace rtc {
 

rtc_base/rtccertificategenerator.cc

@@ -14,6 +14,7 @@
 #include <memory>
 
 #include "rtc_base/checks.h"
+#include "rtc_base/refcountedobject.h"
 #include "rtc_base/sslidentity.h"
 
 namespace rtc {

rtc_base/rtccertificategenerator_unittest.cc

@@ -16,6 +16,7 @@
 #include "rtc_base/checks.h"
 #include "rtc_base/gunit.h"
 #include "rtc_base/logging.h"
+#include "rtc_base/refcountedobject.h"
 #include "rtc_base/thread.h"
 
 namespace rtc {

rtc_base/weak_ptr.h

@@ -16,6 +16,7 @@
 #include <utility>
 
 #include "rtc_base/refcount.h"
+#include "rtc_base/refcountedobject.h"
 #include "rtc_base/scoped_ref_ptr.h"
 #include "rtc_base/sequenced_task_checker.h"