Disable zoom bubble for PDF extension.

extensions/common/features/behavior_feature.cc

 // 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.
 
 #include "extensions/common/features/behavior_feature.h"
 
 namespace extensions {
 
 const char* const BehaviorFeature::kServiceWorker = "service_worker";
 
 const char* const BehaviorFeature::kWhitelistedForIncognito =
     "whitelisted_for_incognito";
 
 const char* const BehaviorFeature::kDoNotSync = "do_not_sync";
 
+const char* const BehaviorFeature::kZoomWithoutBubble = "zoom_without_bubble";
+
 }  // namespace extensions