[Extensions] Convert some ChromeSyncExtensionFunctions

extensions/browser/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 EXTENSIONS_BROWSER_EXTENSION_FUNCTION_H_
 #define EXTENSIONS_BROWSER_EXTENSION_FUNCTION_H_
 
 #include <stddef.h>
 
 #include <list>
@@ skipping 150 matching lines @@
 
   // Helper class for tests to force all ExtensionFunction::user_gesture()
   // calls to return true as long as at least one instance of this class
   // exists.
   class ScopedUserGestureForTests {
    public:
     ScopedUserGestureForTests();
     ~ScopedUserGestureForTests();
   };
 
+  // A string used in the case of an unknown error being detected.
+  // DON'T USE THIS. It's only here during conversion to flag cases where errors
+  // aren't already set.
+  // TODO(devlin): Remove this if/when all functions are updated to return real
+  // errors.
+  static const char* kUnknownErrorDoNotUse;
+
   // Called before Run() in order to perform a common verification check so that
   // APIs subclassing this don't have to roll their own RunSafe() variants.
   // If this returns false, then Run() is never called, and the function
   // responds immediately with an error (note that error must be non-empty in
   // this case). If this returns true, execution continues on to Run().
   virtual bool PreRunValidation(std::string* error);
 
   // Runs the extension function if PreRunValidation() succeeds.
   ResponseAction RunWithValidation();
 
@@ skipping 474 matching lines @@
  private:
   // If you're hitting a compile error here due to "final" - great! You're
   // doing the right thing, you just need to extend UIThreadExtensionFunction
   // instead of AsyncExtensionFunction.
   ResponseAction Run() final;
 
   DISALLOW_COPY_AND_ASSIGN(AsyncExtensionFunction);
 };
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_FUNCTION_H_