Ozone Linux: Fix linker errors in headless_unittests

headless/public/util/error_reporter.h

Index: headless/public/util/error_reporter.h
diff --git a/headless/public/util/error_reporter.h b/headless/public/util/error_reporter.h
index 0f496c21723194b3da8b4775e128a905b0f2f6fa..2fd432f3f2dd4ee51d58aca921300648cb9f99cf 100644
--- a/headless/public/util/error_reporter.h
+++ b/headless/public/util/error_reporter.h
@@ -45,6 +45,7 @@ class HEADLESS_EXPORT ErrorReporter {
   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: