Add "additionalCommands" support to DriverCommandExecutor.

Keeping it protected as there's no existing use case for wider visibility.
Note that I opted for the more standard order of parameters
(for the overloaded methods of this sort) over that unfortunate
non-standard ordering that HttpCommandExecutor uses for some reason.

java/client/src/org/openqa/selenium/remote/HttpCommandExecutor.java

@@ -58,6 +58,10 @@ public HttpCommandExecutor(URL addressOfRemoteServer) {
     this(ImmutableMap.<String, CommandInfo>of(), addressOfRemoteServer);
   }
 
+  /**
+   * Creates an {@link HttpCommandExecutor} that supports non-standard
+   * {@code additionalCommands} in addition to the standard.
+   */
   public HttpCommandExecutor(
       Map<String, CommandInfo> additionalCommands, URL addressOfRemoteServer) {
     this(additionalCommands, addressOfRemoteServer, getDefaultClientFactory());

java/client/src/org/openqa/selenium/remote/service/DriverCommandExecutor.java

@@ -21,12 +21,14 @@
 
 import org.openqa.selenium.WebDriverException;
 import org.openqa.selenium.remote.Command;
+import org.openqa.selenium.remote.CommandInfo;
 import org.openqa.selenium.remote.DriverCommand;
 import org.openqa.selenium.remote.HttpCommandExecutor;
 import org.openqa.selenium.remote.Response;
 
 import java.io.IOException;
 import java.net.ConnectException;
+import java.util.Map;
 
 /**
  * A specialized {@link HttpCommandExecutor} that will use a {@link DriverService} that lives
@@ -48,6 +50,16 @@ public DriverCommandExecutor(DriverService service) {
     this.service = service;
   }
 
+  /**
+   * Creates an {@link DriverCommandExecutor} that supports non-standard
+   * {@code additionalCommands} in addition to the standard.
+   */
+  protected DriverCommandExecutor(
+      DriverService service, Map<String, CommandInfo> additionalCommands) {
+    super(additionalCommands, service.getUrl());
+    this.service = service;
+  }
+
   /**
    * Sends the {@code command} to the driver server for execution. The server will be started
    * if requesting a new session. Likewise, if terminating a session, the server will be shutdown