[iOS] WeakNSObject assignment is consistent with copy constructor

The copy constructor was changed (in CL
https://codereview.chromium.org/853503002/) to allow a WeakNSObject to
be copied on a different thread.
This CL is making the assignment operator behave consistently with the
copy constructor: it is now allowed to assign a WeakNSObject on a
different thread.

In any case, dereferencing the WeakNSObject on another thread remains
disallowed.

BUG=394008

Review URL: https://codereview.chromium.org/1014813002

Cr-Commit-Position: refs/heads/master@{#320915}

base/ios/weak_nsobject.h

@@ -117,7 +117,9 @@ class WeakNSProtocol {
   }
 
   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;
   }

base/ios/weak_nsobject_unittest.mm

@@ -109,8 +109,12 @@ void TouchWeakData(const WeakNSObject<NSMutableData>& weak_data) {
 // the weak object on its original thread.
 void CopyWeakNSObjectAndPost(const WeakNSObject<NSMutableData>& weak_object,
                              scoped_refptr<SingleThreadTaskRunner> runner) {
-  WeakNSObject<NSMutableData> weak_copy(weak_object);
-  runner->PostTask(FROM_HERE, Bind(&TouchWeakData, weak_copy));
+  // Copy using constructor.
+  WeakNSObject<NSMutableData> weak_copy1(weak_object);
+  runner->PostTask(FROM_HERE, Bind(&TouchWeakData, weak_copy1));
+  // Copy using assignment operator.
+  WeakNSObject<NSMutableData> weak_copy2 = weak_object;
+  runner->PostTask(FROM_HERE, Bind(&TouchWeakData, weak_copy2));
 }
 
 // Tests that the weak object can be copied on a different thread.
@@ -128,8 +132,8 @@ TEST(WeakNSObjectTest, WeakNSObjectCopyOnOtherThread) {
   other_thread.Stop();
   loop.RunUntilIdle();
 
-  // Check that TouchWeakData was called.
-  EXPECT_EQ(1u, [data length]);
+  // Check that TouchWeakData was called and the object touched twice.
+  EXPECT_EQ(2u, [data length]);
 }
 
 }  // namespace