Update from https://crrev.com/320931

base/ios/weak_nsobject.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_IOS_WEAK_NSOBJECT_H_
 #define BASE_IOS_WEAK_NSOBJECT_H_
 
 #import <Foundation/Foundation.h>
 #import <objc/runtime.h>
 
@@ skipping 99 matching lines @@
   }
 
   NST get() const {
     DCHECK(checker_.CalledOnValidThread());
     if (!container_.get())
       return nil;
     return container_->object();
   }
 
   WeakNSProtocol& operator=(const WeakNSProtocol<NST>& that) {
-    DCHECK(checker_.CalledOnValidThread());
+    // A WeakNSProtocol object can be copied on one thread and used on
+    // another.
+    checker_.DetachFromThread();
     container_ = that.container_;
     return *this;
   }
 
   bool operator==(NST that) const {
     DCHECK(checker_.CalledOnValidThread());
     return get() == that;
   }
 
   bool operator!=(NST that) const {
@@ skipping 47 matching lines @@
 
   WeakNSObject& operator=(const WeakNSObject<id>& that) {
     WeakNSProtocol<id>::operator=(that);
     return *this;
   }
 };
 
 }  // namespace base
 
 #endif  // BASE_IOS_WEAK_NSOBJECT_H_