[Extensions] Move Feature channel code into //extensions

extensions/common/features/feature.h

Index: extensions/common/features/feature.h
diff --git a/extensions/common/features/feature.h b/extensions/common/features/feature.h
index b9f8073a020ea0d7b0028ded400f033ac75d699e..110cb48b9def246173f55e61d0ca159edb1f8570 100644
--- a/extensions/common/features/feature.h
+++ b/extensions/common/features/feature.h
@@ -141,11 +141,6 @@ class Feature {
                                             const GURL& url,
                                             Platform platform) const = 0;
 
-  virtual std::string GetAvailabilityMessage(AvailabilityResult result,
-                                             Manifest::Type type,
-                                             const GURL& url,
-                                             Context context) const = 0;
-
   // Returns true if the feature is available to the current environment,
   // without needing to know information about an Extension or any other
   // contextual information. Typically used when the Feature is purely