Add Locking calls to file_io.h plus implementations and test

util/test/errors.h

Index: util/test/errors.h
diff --git a/util/test/errors.h b/util/test/errors.h
index 366acf1fe01e1f0e114e5cad0f43fdbb3deb1cc4..c1d79bf92e682ff34eaf44463c8aa49c19f3b021 100644
--- a/util/test/errors.h
+++ b/util/test/errors.h
@@ -17,6 +17,8 @@
 
 #include <string>
 
+#include "build/build_config.h"
+
 namespace crashpad {
 namespace test {
 
@@ -65,6 +67,15 @@ std::string ErrnoMessage(int err, const std::string& base = std::string());
 //!     a colon.
 std::string ErrnoMessage(const std::string& base = std::string());
 
+#if defined(OS_WIN) || DOXYGEN
+//! \brief Formats an error message using `GetLastError()`.
+//!
+//! The returned string will combine the \a base string, if supplied, with a
+//! a textual and numeric description of the error. The format is the same as
+//! the `PLOG()` formatting in base.
+std::string ErrorMessage(const std::string& base = std::string());
+#endif
+
 }  // namespace test
 }  // namespace crashpad