[Extensions] Add a SimpleFeature::Validate function

extensions/common/features/manifest_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/manifest_feature.h"
 
 #include "extensions/common/manifest.h"
 
 namespace extensions {
 
@@ skipping 16 matching lines @@
     return availability;
 
   // We know we can skip manifest()->GetKey() here because we just did the same
   // validation it would do above.
   if (extension && !extension->manifest()->value()->HasKey(name()))
     return CreateAvailability(NOT_PRESENT, extension->GetType());
 
   return CreateAvailability(IS_AVAILABLE);
 }
 
-std::string ManifestFeature::Parse(const base::DictionaryValue* value) {
-  std::string error = SimpleFeature::Parse(value);
-  if (!error.empty())
-    return error;
+bool ManifestFeature::Validate(std::string* error) {
+  if (!SimpleFeature::Validate(error))
+    return false;
 
   if (extension_types()->empty()) {
-    return name() + ": Manifest features must specify at least one " +
-        "value for extension_types.";
+    *error = name() + ": Manifest features must specify at least one " +
+             "value for extension_types.";
+    return false;
   }
 
-  if (value->HasKey("contexts"))
-    return name() + ": Manifest features do not support contexts.";
+  if (!contexts()->empty()) {
+    *error = name() + ": Manifest features do not support contexts.";
+    return false;
+  }
 
-  return std::string();
+  return true;
 }
 
 }  // namespace extensions