Adding locking for lazy-load of static dictionary in .NET

When running in parallel, there is a chance that multiple threads could
attempt to populate a static dictionary used for translating error codes.
This commit adds locking around the initialization of the static
dictionary. Note that we are not using "double-check" locking, but the
locking we use should be good enough. Fixes and closes issue SeleniumHQ#3166.

dotnet/src/webdriver/Remote/WebDriverError.cs

@@ -179,6 +179,7 @@ internal static class WebDriverError
         public const string UnsupportedOperation = "unsupported operation";
 
         private static Dictionary<string, WebDriverResult> resultMap;
+        private static object lockObject = new object();
 
         /// <summary>
         /// Converts a string error to a <see cref="WebDriverResult"/> value.
@@ -187,9 +188,12 @@ internal static class WebDriverError
         /// <returns>The converted <see cref="WebDriverResult"/> value.</returns>
         public static WebDriverResult ResultFromError(string error)
         {
-            if (resultMap == null)
+            lock(lockObject)
             {
-                InitializeResultMap();
+                if (resultMap == null)
+                {
+                    InitializeResultMap();
+                }
             }
 
             if (!resultMap.ContainsKey(error))