[Extensions] Validate file existence for chrome url overrides

chrome/common/extensions/chrome_manifest_url_handlers.h

 // Copyright (c) 2012 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_COMMON_EXTENSIONS_CHROME_MANIFEST_URL_HANDLERS_H_
 #define CHROME_COMMON_EXTENSIONS_CHROME_MANIFEST_URL_HANDLERS_H_
 
 #include <string>
 
 #include "base/macros.h"
@@ skipping 41 matching lines @@
   DISALLOW_COPY_AND_ASSIGN(DevToolsPageHandler);
 };
 
 // Parses the "chrome_url_overrides" manifest key.
 class URLOverridesHandler : public ManifestHandler {
  public:
   URLOverridesHandler();
   ~URLOverridesHandler() override;
 
   bool Parse(Extension* extension, base::string16* error) override;
+  bool Validate(const Extension* extension,
+                std::string* error,
+                std::vector<InstallWarning>* warnings) const override;
 
  private:
   const std::vector<std::string> Keys() const override;
 
   DISALLOW_COPY_AND_ASSIGN(URLOverridesHandler);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_COMMON_EXTENSIONS_CHROME_MANIFEST_URL_HANDLERS_H_