Update from https://crrev.com/308331

base/ios/weak_nsobject.h

Index: base/ios/weak_nsobject.h
diff --git a/base/ios/weak_nsobject.h b/base/ios/weak_nsobject.h
new file mode 100644
index 0000000000000000000000000000000000000000..46aecb5e0b641fb3d004e23b71a5111f02295bfe
--- /dev/null
+++ b/base/ios/weak_nsobject.h
@@ -0,0 +1,166 @@
+// 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>
+
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "base/logging.h"
+#include "base/memory/ref_counted.h"
+#include "base/threading/non_thread_safe.h"
+#include "base/threading/thread_checker.h"
+
+// WeakNSObject<> is patterned after scoped_nsobject<>, but instead of
+// maintaining ownership of an NSObject subclass object, it will nil itself out
+// when the object is deallocated.
+//
+// WeakNSProtocol<> has the same behavior as WeakNSObject, but can be used
+// with protocols.
+//
+// Example usage (base::WeakNSObject<T>):
+//   scoped_nsobject<Foo> foo([[Foo alloc] init]);
+//   WeakNSObject<Foo> weak_foo;  // No pointer
+//   weak_foo.reset(foo)  // Now a weak reference is kept.
+//   [weak_foo description];  // Returns [foo description].
+//   foo.reset();  // The reference is released.
+//   [weak_foo description];  // Returns nil, as weak_foo is pointing to nil.
+//
+//
+// Implementation wise a WeakNSObject keeps a reference to a refcounted
+// WeakContainer. There is one unique instance of a WeakContainer per watched
+// NSObject, this relationship is maintained via the ObjectiveC associated
+// object API, indirectly via an ObjectiveC CRBWeakNSProtocolSentinel class.
+//
+// The implementation assumes that the tracked object will be released on the
+// same thread that the WeakNSObject is created on.
+//
+namespace base {
+
+// WeakContainer keeps a weak pointer to an object and clears it when it
+// receives nullify() from the object's sentinel.
+class WeakContainer : public base::RefCountedThreadSafe<WeakContainer> {
+ public:
+  WeakContainer(id object) : object_(object) {}
+  id object() { return object_; }
+  void nullify() {
+    DCHECK(checker_.CalledOnValidThread());
+    object_ = nil;
+  }
+
+ private:
+  friend base::RefCountedThreadSafe<WeakContainer>;
+  ~WeakContainer() {}
+  base::ThreadChecker checker_;
+  id object_;
+};
+
+}  // namespace base
+
+// Sentinel for observing the object contained in the weak pointer. The object
+// will be deleted when the weak object is deleted and will notify its
+// container.
+@interface CRBWeakNSProtocolSentinel : NSObject
+// Return the only associated container for this object. There can be only one.
+// Will return null if object is nil .
++ (scoped_refptr<base::WeakContainer>)containerForObject:(id)object;
+@end
+
+namespace base {
+
+// Base class for all WeakNSObject derivatives.
+template <typename NST>
+class WeakNSProtocol : public base::NonThreadSafe {
+ public:
+  explicit WeakNSProtocol(NST object = nil) {
+    container_ = [CRBWeakNSProtocolSentinel containerForObject:object];
+  }
+
+  WeakNSProtocol(const WeakNSProtocol<NST>& that) {
+    container_ = that.container_;
+  }
+
+  ~WeakNSProtocol() {
+    // A WeakNSProtocol object can be allocated on one thread and released on
+    // another. This is not the case for the contained object.
+    DetachFromThread();
+  }
+
+  void reset(NST object = nil) {
+    DCHECK(CalledOnValidThread());
+    container_ = [CRBWeakNSProtocolSentinel containerForObject:object];
+  }
+
+  NST get() const {
+    DCHECK(CalledOnValidThread());
+    if (!container_.get())
+      return nil;
+    return container_->object();
+  }
+
+  WeakNSProtocol& operator=(const WeakNSProtocol<NST>& that) {
+    DCHECK(CalledOnValidThread());
+    container_ = that.container_;
+    return *this;
+  }
+
+  bool operator==(NST that) const {
+    DCHECK(CalledOnValidThread());
+    return get() == that;
+  }
+
+  bool operator!=(NST that) const { return get() != that; }
+
+  operator NST() const { return get(); }
+
+ private:
+  // Refecounted reference to the container tracking the ObjectiveC object this
+  // class encapsulates.
+  scoped_refptr<base::WeakContainer> container_;
+};
+
+// Free functions
+template <class NST>
+bool operator==(NST p1, const WeakNSProtocol<NST>& p2) {
+  return p1 == p2.get();
+}
+
+template <class NST>
+bool operator!=(NST p1, const WeakNSProtocol<NST>& p2) {
+  return p1 != p2.get();
+}
+
+template <typename NST>
+class WeakNSObject : public WeakNSProtocol<NST*> {
+ public:
+  explicit WeakNSObject(NST* object = nil) : WeakNSProtocol<NST*>(object) {}
+
+  WeakNSObject(const WeakNSObject<NST>& that) : WeakNSProtocol<NST*>(that) {}
+
+  WeakNSObject& operator=(const WeakNSObject<NST>& that) {
+    WeakNSProtocol<NST*>::operator=(that);
+    return *this;
+  }
+};
+
+// Specialization to make WeakNSObject<id> work.
+template <>
+class WeakNSObject<id> : public WeakNSProtocol<id> {
+ public:
+  explicit WeakNSObject(id object = nil) : WeakNSProtocol<id>(object) {}
+
+  WeakNSObject(const WeakNSObject<id>& that) : WeakNSProtocol<id>(that) {}
+
+  WeakNSObject& operator=(const WeakNSObject<id>& that) {
+    WeakNSProtocol<id>::operator=(that);
+    return *this;
+  }
+};
+
+}  // namespace base
+
+#endif  // BASE_IOS_WEAK_NSOBJECT_H_