Correct iOS build for latest Xcode beta.

ios/web/weak_nsobject_counter.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 "ios/web/weak_nsobject_counter.h"
 
 #import <objc/runtime.h>
 
 #include "base/logging.h"
 #import "base/mac/scoped_nsobject.h"
@@ skipping 11 matching lines @@
 // Designated initializer. |object| cannot be nil. It registers self as an
 // associated object to |object|.
 - (instancetype)initWithSharedCounter:(const linked_ptr<NSUInteger>&)counter
                    objectToBeObserved:(id)object;
 @end
 
 @implementation CRBWeakNSObjectDeallocationObserver {
   linked_ptr<NSUInteger> _counter;
 }
 
-- (instancetype)init {
-  NOTREACHED();
-  return nil;
-}
-
 - (instancetype)initWithSharedCounter:(const linked_ptr<NSUInteger>&)counter
                    objectToBeObserved:(id)object {
   self = [super init];
   if (self) {
     DCHECK(counter.get());
     DCHECK(object);
     _counter = counter;
     objc_setAssociatedObject(object, &kObserverAssociatedObjectKey, self,
                              OBJC_ASSOCIATION_RETAIN);
     (*_counter)++;
   }
   return self;
 }
 
+- (instancetype)init {
+  NOTREACHED();
+  return nil;
+}
+
 - (void)dealloc {
   DCHECK(_counter.get());
   (*_counter)--;
   _counter.reset();
   [super dealloc];
 }
 
 @end
 
 namespace web {
@@ skipping 13 matching lines @@
       [[CRBWeakNSObjectDeallocationObserver alloc]
           initWithSharedCounter:counter_ objectToBeObserved:object]);
 }
 
 NSUInteger WeakNSObjectCounter::Size() const {
   DCHECK(CalledOnValidThread());
   return *counter_;
 }
 
 }  // namespace web