Adds move assignment operator to scoped_refptr

Implementation borrowed from chromium.

BUG=webrtc:5556
R=pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/2193443002 .

Cr-Commit-Position: refs/heads/master@{#13577}

webrtc/base/scoped_ref_ptr.h

@@ -63,7 +63,7 @@
 #ifndef WEBRTC_BASE_SCOPED_REF_PTR_H_
 #define WEBRTC_BASE_SCOPED_REF_PTR_H_
 
-#include <stddef.h>
+#include <memory>
 
 namespace rtc {
 
@@ -134,6 +134,17 @@ class scoped_refptr {
     return *this = r.get();
   }
 
+  scoped_refptr<T>& operator=(scoped_refptr<T>&& r) {
+    scoped_refptr<T>(std::move(r)).swap(*this);
+    return *this;
+  }
+
+  template <typename U>
+  scoped_refptr<T>& operator=(scoped_refptr<U>&& r) {
+    scoped_refptr<T>(std::move(r)).swap(*this);
+    return *this;
+  }
+
   void swap(T** pp) {
     T* p = ptr_;
     ptr_ = *pp;