Update from https://crrev.com/308331

base/ios/weak_nsobject.mm

Index: base/ios/weak_nsobject.mm
diff --git a/base/ios/weak_nsobject.mm b/base/ios/weak_nsobject.mm
new file mode 100644
index 0000000000000000000000000000000000000000..36f9d3ea7cdd700da53d77a34245c3bad68a86c1
--- /dev/null
+++ b/base/ios/weak_nsobject.mm
@@ -0,0 +1,61 @@
+// 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.
+
+#include "base/ios/weak_nsobject.h"
+
+#include "base/mac/scoped_nsautorelease_pool.h"
+#include "base/mac/scoped_nsobject.h"
+
+namespace {
+// The key needed by objc_setAssociatedObject.
+char sentinelObserverKey_;
+}
+
+@interface CRBWeakNSProtocolSentinel ()
+// Container to notify on dealloc.
+@property(readonly, assign) scoped_refptr<base::WeakContainer> container;
+// Designed initializer.
+- (id)initWithContainer:(scoped_refptr<base::WeakContainer>)container;
+@end
+
+@implementation CRBWeakNSProtocolSentinel
+
+@synthesize container = container_;
+
++ (scoped_refptr<base::WeakContainer>)containerForObject:(id)object {
+  if (object == nil)
+    return nullptr;
+  // The autoreleasePool is needed here as the call to objc_getAssociatedObject
+  // returns an autoreleased object which is better released sooner than later.
+  base::mac::ScopedNSAutoreleasePool pool;
+  CRBWeakNSProtocolSentinel* sentinel =
+      objc_getAssociatedObject(object, &sentinelObserverKey_);
+  if (!sentinel) {
+    base::scoped_nsobject<CRBWeakNSProtocolSentinel> newSentinel(
+        [[CRBWeakNSProtocolSentinel alloc]
+            initWithContainer:new base::WeakContainer(object)]);
+    sentinel = newSentinel;
+    objc_setAssociatedObject(object, &sentinelObserverKey_, sentinel,
+                             OBJC_ASSOCIATION_RETAIN);
+    // The retain count is 2. One retain is due to the alloc, the other to the
+    // association with the weak object.
+    DCHECK_EQ(2u, [sentinel retainCount]);
+  }
+  return [sentinel container];
+}
+
+- (id)initWithContainer:(scoped_refptr<base::WeakContainer>)container {
+  DCHECK(container.get());
+  self = [super init];
+  if (self)
+    container_ = container;
+  return self;
+}
+
+- (void)dealloc {
+  self.container->nullify();
+  [super dealloc];
+}
+
+@end