Correct iOS build for latest Xcode beta.

ios/web/weak_nsobject_counter.mm

Index: ios/web/weak_nsobject_counter.mm
diff --git a/ios/web/weak_nsobject_counter.mm b/ios/web/weak_nsobject_counter.mm
index bd14b7b3f0180ae2cd6d7d7dd95883617d343be8..edcf01ae1ac7f34c66b8e49371449a1f499a38c9 100644
--- a/ios/web/weak_nsobject_counter.mm
+++ b/ios/web/weak_nsobject_counter.mm
@@ -29,11 +29,6 @@ const char kObserverAssociatedObjectKey = 'h';
   linked_ptr<NSUInteger> _counter;
 }
 
-- (instancetype)init {
-  NOTREACHED();
-  return nil;
-}
-
 - (instancetype)initWithSharedCounter:(const linked_ptr<NSUInteger>&)counter
                    objectToBeObserved:(id)object {
   self = [super init];
@@ -48,6 +43,11 @@ const char kObserverAssociatedObjectKey = 'h';
   return self;
 }
 
+- (instancetype)init {
+  NOTREACHED();
+  return nil;
+}
+
 - (void)dealloc {
   DCHECK(_counter.get());
   (*_counter)--;