[Extensions Bindings] Add a GetStringPropertyFromObject test method

extensions/renderer/api_binding_test_util.h

 // Copyright 2016 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 EXTENSIONS_RENDERER_API_BINDING_TEST_UTIL_H_
 #define EXTENSIONS_RENDERER_API_BINDING_TEST_UTIL_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 88 matching lines @@
 v8::Local<v8::Value> GetPropertyFromObject(v8::Local<v8::Object> object,
                                            v8::Local<v8::Context> context,
                                            base::StringPiece key);
 
 // As above, but converts the result to a base::Value.
 std::unique_ptr<base::Value> GetBaseValuePropertyFromObject(
     v8::Local<v8::Object> object,
     v8::Local<v8::Context> context,
     base::StringPiece key);
 
+// As above, but returns a JSON-serialized version of the value, or
+// "undefined", "null", "function", or "empty".
+std::string GetStringPropertyFromObject(v8::Local<v8::Object> object,
+                                        v8::Local<v8::Context> context,
+                                        base::StringPiece key);
+
 }  // extensions
 
 #endif  // EXTENSIONS_RENDERER_API_BINDING_TEST_UTIL_H_