Adds empty methods to Mutex on platforms where Google Test is not thread-safe. This will support a reentrancy fix in Google Mock.

include/gtest/internal/gtest-port.h

@@ -1416,6 +1416,8 @@ class ThreadLocal {
 class Mutex {
  public:
   Mutex() {}
+  void Lock() {}
+  void Unlock() {}
   void AssertHeld() const {}
 };