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 9d8c539414bb42f775911dbd76fa7c1cbaed192f..5ec95cf91b4a55e2714bcb176e960f4a29cdbe11 100644
--- a/extensions/common/features/simple_feature.h
+++ b/extensions/common/features/simple_feature.h
@@ -29,6 +29,18 @@ class SimpleFeatureTest;
 
 class SimpleFeature : public Feature {
  public:
+  // Used by tests to override the cached --whitelisted-extension-id.
+  class ScopedWhitelistForTest {
+   public:
+    explicit ScopedWhitelistForTest(const std::string& id);
+    ~ScopedWhitelistForTest();
+
+   private:
+    std::string* previous_id_;
+
+    DISALLOW_COPY_AND_ASSIGN(ScopedWhitelistForTest);
+  };
+
   SimpleFeature();
   ~SimpleFeature() override;