Implement CopyOnWriteBuffer move using scoped_refptr move

webrtc/base/copyonwritebuffer.h

Index: webrtc/base/copyonwritebuffer.h
diff --git a/webrtc/base/copyonwritebuffer.h b/webrtc/base/copyonwritebuffer.h
index a7e52beea53c3ce442ac3b97a1db013590e08861..16e15f946b186e71f460fe36c3ab2a21597a11b7 100644
--- a/webrtc/base/copyonwritebuffer.h
+++ b/webrtc/base/copyonwritebuffer.h
@@ -119,9 +119,7 @@ class CopyOnWriteBuffer {
   CopyOnWriteBuffer& operator=(CopyOnWriteBuffer&& buf) {
     RTC_DCHECK(IsConsistent());
     RTC_DCHECK(buf.IsConsistent());
-    // TODO(jbauch): use std::move once scoped_refptr supports it (issue 5556)
-    buffer_.swap(buf.buffer_);
-    buf.buffer_ = nullptr;
+    buffer_ = std::move(buf.buffer_);
     return *this;
   }