Cache --whitelisted-extension-id in SimpleFeature.

chrome/browser/extensions/test_extension_environment.h

Index: chrome/browser/extensions/test_extension_environment.h
diff --git a/chrome/browser/extensions/test_extension_environment.h b/chrome/browser/extensions/test_extension_environment.h
index 57136588c89aa5bcd32810bf73175deb368820bc..bea77e1d4e96290f959ab27761abbc867032f240 100644
--- a/chrome/browser/extensions/test_extension_environment.h
+++ b/chrome/browser/extensions/test_extension_environment.h
@@ -62,6 +62,11 @@ class TestExtensionEnvironment {
   // manifest_extra override these defaults.
   const Extension* MakeExtension(const base::Value& manifest_extra);
 
+  // Use a specific extension ID instead of the default generated in
+  // Extension::Create.
+  const Extension* MakeExtension(const base::Value& manifest_extra,
+                                 const std::string& id);
+
   // Returns a test web contents that has a tab id.
   scoped_ptr<content::WebContents> MakeTab() const;