Clean up TestFeaturesNativeHandler to use the ExtensionsClient's JSON feature sources.

extensions/common/manifest.cc

Index: extensions/common/manifest.cc
diff --git a/extensions/common/manifest.cc b/extensions/common/manifest.cc
index bbf2ff85e932f73c5d0cf992d57be1f87469d0c1..37880d42f3ba9412470fa8042433249b2c403507 100644
--- a/extensions/common/manifest.cc
+++ b/extensions/common/manifest.cc
@@ -146,7 +146,7 @@ bool Manifest::ValidateManifest(
   // TODO(aa): Consider having an error here in the case of strict error
   // checking to let developers know when they screw up.
 
-  FeatureProvider* manifest_feature_provider =
+  const FeatureProvider* manifest_feature_provider =
       FeatureProvider::GetManifestFeatures();
   const std::vector<std::string>& feature_names =
       manifest_feature_provider->GetAllFeatureNames();