Implement FeatureProvider for ExtensionAPI.

chrome/common/extensions/manifest_feature.cc

+// Copyright (c) 2012 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 "chrome/common/extensions/manifest_feature.h"
+
+#include "chrome/common/extensions/manifest.h"
+
+namespace extensions {
+
+ManifestFeature::ManifestFeature() {
+}
+
+ManifestFeature::~ManifestFeature() {
+}
+
+Feature::Availability ManifestFeature::IsAvailableToContext(
+    const Extension* extension,
+    Feature::Context context,
+    Feature::Platform platform) const {
+  Availability availability = Feature::IsAvailableToContext(extension,
+                                                            context,
+                                                            platform);
+  if (availability != IS_AVAILABLE)
+    return availability;
+
+  // We know we can skip manifest()->GetKey() here because we just did the same
+  // validation it would do above.
+  if (!extension->manifest()->value()->HasKey(name()))
+    return NOT_PRESENT;
+
+  return IS_AVAILABLE;
+}
+
+}  // namespace