Ozone Linux: Fix linker errors in headless_unittests

headless/public/util/error_reporter.h

 // Copyright 2016 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 HEADLESS_PUBLIC_UTIL_ERROR_REPORTER_H_
 #define HEADLESS_PUBLIC_UTIL_ERROR_REPORTER_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 27 matching lines @@
   bool HasErrors() const;
 
   // Returns a list of reported errors.
   const std::vector<std::string>& errors() const { return errors_; }
 #else  // DCHECK_IS_ON()
   inline void Push() {}
   inline void Pop() {}
   inline void SetName(const char* name) {}
   inline void AddError(base::StringPiece description) {}
   inline bool HasErrors() const { return false; }
+  std::vector<std::string> errors() const { return std::vector<std::string>(); }
 #endif  // DCHECK_IS_ON()
 
  private:
   std::vector<const char*> path_;
   std::vector<std::string> errors_;
 };
 
 }  // namespace headless
 
 #endif  // HEADLESS_PUBLIC_UTIL_ERROR_REPORTER_H_