Add browser controller and allow it to be used under a flag.

tools/testing/dart/test_options.dart

Index: tools/testing/dart/test_options.dart
===================================================================
--- tools/testing/dart/test_options.dart	(revision 22503)
+++ tools/testing/dart/test_options.dart	(working copy)
@@ -301,6 +301,14 @@
               [],
               false,
               'bool'
+              ),
+          new _TestOptionSpecification(
+              'use_browser_controller',
+              'Use the new selenium replacement browser-controller.',
+              ['--use_browser_controller'],
+              [],
+              false,
+              'bool'
               ),];
   }