Implemented AddAllocWithZoneMethod to override allocWithZone: on demand.

ios/web/alloc_with_zone_interceptor.mm

Index: ios/web/alloc_with_zone_interceptor.mm
diff --git a/ios/web/alloc_with_zone_interceptor.mm b/ios/web/alloc_with_zone_interceptor.mm
new file mode 100644
index 0000000000000000000000000000000000000000..ea86016561ec1c76d6104a93b98ec7afaafbded7
--- /dev/null
+++ b/ios/web/alloc_with_zone_interceptor.mm
@@ -0,0 +1,27 @@
+// Copyright 2015 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/alloc_with_zone_interceptor.h"
+
+#import <objc/runtime.h>
+
+#include "base/logging.h"
+
+namespace web {
+
+void AddAllocWithZoneMethod(Class target, id (^impl_block)(Class, NSZone*)) {
+  // Make sure |allocWithZone:| is not already implemented in the target class.
+  Class meta_class = object_getClass(target);
+  DCHECK_EQ(
+      class_getMethodImplementation(meta_class, @selector(allocWithZone:)),
+      class_getMethodImplementation(object_getClass([NSObject class]),
+                                    @selector(allocWithZone:)));
+
+  IMP new_impl = imp_implementationWithBlock(^(id self, NSZone* zone) {
+    return impl_block(self, zone);
+  });
+  class_addMethod(meta_class, @selector(allocWithZone:), new_impl, "v@:@");
+}
+
+}  // namespace web