Cache --whitelisted-extension-id in SimpleFeature.

extensions/common/features/simple_feature.h

Index: extensions/common/features/simple_feature.h
diff --git a/extensions/common/features/simple_feature.h b/extensions/common/features/simple_feature.h
index a5bc27080d0b88a1b64d061c4c1838692ae3e29e..16c9e2fbf3ab4ccc9249e00cc55f1b972af7888c 100644
--- a/extensions/common/features/simple_feature.h
+++ b/extensions/common/features/simple_feature.h
@@ -139,6 +139,10 @@ class SimpleFeature : public Feature {
   Availability CheckDependencies(
       const base::Callback<Availability(const Feature*)>& checker) const;
 
+  // A singleton copy of the --whitelisted-extension-id so that we don't need to
+  // copy it from the CommandLine each time.
+  static const std::string& whitelisted_extension_id();
+
   // For clarity and consistency, we handle the default value of each of these
   // members the same way: it matches everything. It is up to the higher level
   // code that reads Features out of static data to validate that data and set