[Extensions] Add a SimpleFeature::Validate function

extensions/common/features/permission_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/permission_feature.h"
 
 #include "extensions/common/permissions/permission_set.h"
 #include "extensions/common/permissions/permissions_data.h"
 
 namespace extensions {
@@ skipping 15 matching lines @@
                                                                   platform);
   if (!availability.is_available())
     return availability;
 
   if (extension && !extension->permissions_data()->HasAPIPermission(name()))
     return CreateAvailability(NOT_PRESENT, extension->GetType());
 
   return CreateAvailability(IS_AVAILABLE);
 }
 
-std::string PermissionFeature::Parse(const base::DictionaryValue* value) {
-  std::string error = SimpleFeature::Parse(value);
-  if (!error.empty())
-    return error;
+bool PermissionFeature::Validate(std::string* error) {
+  if (!SimpleFeature::Validate(error))
+    return false;
 
   if (extension_types()->empty()) {
-    return name() + ": Permission features must specify at least one " +
-        "value for extension_types.";
+    *error = name() + ": Permission features must specify at least one " +
+             "value for extension_types.";
+    return false;
   }
 
-  if (value->HasKey("contexts"))
-    return name() + ": Permission features do not support contexts.";
+  if (!contexts()->empty()) {
+    *error = name() + ": Permission features do not support contexts.";
+    return false;
+  }
 
-  return std::string();
+  return true;
 }
 
 }  // namespace extensions