The Java Client should be aware of the readback status.

content/public/browser/readback_types.h

 // 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.
 
 #ifndef CONTENT_COMMON_READBACK_TYPES_H_
 #define CONTENT_COMMON_READBACK_TYPES_H_
 
 #include "base/callback.h"
 
 class SkBitmap;
 
 namespace content {
 
+// ReadbackResponse type used in ContentReadbackHandler.
+//
+// A Java counterpart will be generated for this enum.
+// GENERATED_JAVA_ENUM_PACKAGE: (
+//   org.chromium.content_public.browser.readback_types)
+// GENERATED_JAVA_PREFIX_TO_STRIP: READBACK_
 enum ReadbackResponse {
   READBACK_SUCCESS,
   READBACK_FAILED,
   READBACK_FORMAT_NOT_SUPPORTED,
   READBACK_NOT_SUPPORTED,
   READBACK_SURFACE_UNAVAILABLE,
   READBACK_MEMORY_ALLOCATION_FAILURE,
 };
 
 typedef const base::Callback<void(const SkBitmap&, ReadbackResponse)>
     ReadbackRequestCallback;
 
 }  // namespace content
 
 #endif  // CONTENT_COMMON_READBACK_TYPES_H_