Update from https://crrev.com/316786

base/ios/crb_protocol_observers.mm

+// Copyright 2014 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.
+
+#import "base/ios/crb_protocol_observers.h"
+
+#include <objc/runtime.h>
+
+#include "base/logging.h"
+#include "base/mac/scoped_nsobject.h"
+
+@interface CRBProtocolObservers ()
+
+// Designated initializer.
+- (id)initWithProtocol:(Protocol*)protocol;
+
+@end
+
+@implementation CRBProtocolObservers {
+  base::scoped_nsobject<Protocol> _protocol;
+  base::scoped_nsobject<NSHashTable> _observers;
+}
+
++ (CRBProtocolObservers*)observersWithProtocol:(Protocol*)protocol {
+  return [[[self alloc] initWithProtocol:protocol] autorelease];
+}
+
+- (id)init {
+  NOTREACHED();
+  return nil;
+}
+
+- (id)initWithProtocol:(Protocol*)protocol {
+  self = [super init];
+  if (self) {
+    _protocol.reset([protocol retain]);
+    _observers.reset([[NSHashTable weakObjectsHashTable] retain]);
+  }
+  return self;
+}
+
+- (Protocol*)protocol {
+  return _protocol.get();
+}
+
+- (void)addObserver:(id)observer {
+  DCHECK([observer conformsToProtocol:self.protocol]);
+  [_observers addObject:observer];
+}
+
+- (void)removeObserver:(id)observer {
+  [_observers removeObject:observer];
+}
+
+#pragma mark - NSObject
+
+- (NSMethodSignature*)methodSignatureForSelector:(SEL)selector {
+  NSMethodSignature* signature = [super methodSignatureForSelector:selector];
+  if (signature)
+    return signature;
+
+  // Look for a required method in the protocol. protocol_getMethodDescription
+  // returns a struct whose fields are null if a method for the selector was
+  // not found.
+  struct objc_method_description description =
+      protocol_getMethodDescription(self.protocol, selector, YES, YES);
+  if (description.types)
+    return [NSMethodSignature signatureWithObjCTypes:description.types];
+
+  // Look for an optional method in the protocol.
+  description = protocol_getMethodDescription(self.protocol, selector, NO, YES);
+  if (description.types)
+    return [NSMethodSignature signatureWithObjCTypes:description.types];
+
+  // There is neither a required nor optional method with this selector in the
+  // protocol, so invoke -[NSObject doesNotRecognizeSelector:] to raise
+  // NSInvalidArgumentException.
+  [self doesNotRecognizeSelector:selector];
+  return nil;
+}
+
+- (void)forwardInvocation:(NSInvocation*)invocation {
+  SEL selector = [invocation selector];
+  base::scoped_nsobject<NSArray> observers([[_observers allObjects] retain]);
+  for (id observer in observers.get()) {
+    if ([observer respondsToSelector:selector])
+      [invocation invokeWithTarget:observer];
+  }
+}
+
+- (void)executeOnObservers:(ExecutionWithObserverBlock)callback {
+  DCHECK(callback);
+  base::scoped_nsobject<NSArray> observers([[_observers allObjects] retain]);
+  for (id observer in observers.get()) {
+    callback(observer);
+  }
+}
+
+@end