[Extensions] Apply WARN_UNUSED_RESULT and final keyword to ExtensionFunctions.

chrome/browser/extensions/chrome_extension_function.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_CHROME_EXTENSION_FUNCTION_H_
 #define CHROME_BROWSER_EXTENSIONS_CHROME_EXTENSION_FUNCTION_H_
 
 #include "extensions/browser/extension_function.h"
 
 class Browser;
@@ skipping 69 matching lines @@
  protected:
   ~ChromeAsyncExtensionFunction() override;
 
   // Deprecated, see AsyncExtensionFunction::RunAsync.
   virtual bool RunAsync() = 0;
 
   // ValidationFailure override to match RunAsync().
   static bool ValidationFailure(ChromeAsyncExtensionFunction* function);
 
  private:
-  ResponseAction Run() override;
+  // If you're hitting a compile error here due to "final" - great! You're doing
+  // the right thing, you just need to extend ChromeUIThreadExtensionFunction
+  // instead of ChromeAsyncExtensionFunction.
+  ResponseAction Run() final;
 };
 
 // A chrome specific analog to SyncExtensionFunction. This has access to a
 // chrome Profile.
 //
 // DEPRECATED: Please consider inherting UIThreadExtensionFunction or
 // SyncExtensionFunction directly. Then if you need access to Chrome details,
 // you can construct a ChromeExtensionFunctionDetails object within your
 // function implementation.
 class ChromeSyncExtensionFunction : public ChromeUIThreadExtensionFunction {
  public:
   ChromeSyncExtensionFunction();
 
  protected:
   ~ChromeSyncExtensionFunction() override;
 
   // Deprecated, see SyncExtensionFunction::RunSync.
   virtual bool RunSync() = 0;
 
   // ValidationFailure override to match RunSync().
   static bool ValidationFailure(ChromeSyncExtensionFunction* function);
 
  private:
-  ResponseAction Run() override;
+  // If you're hitting a compile error here due to "final" - great! You're doing
+  // the right thing, you just need to extend ChromeUIThreadExtensionFunction
+  // instead of ChromeSyncExtensionFunction.
+  ResponseAction Run() final;
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_CHROME_EXTENSION_FUNCTION_H_