Remove the --use_browser_controller flag, now that it is always true.

tools/test.dart

Index: tools/test.dart
diff --git a/tools/test.dart b/tools/test.dart
index 05566625fc61e3e22f1abb368fc8811da8903548..4d7145924f4e6e5026d4896ebe15a10c547fa716 100755
--- a/tools/test.dart
+++ b/tools/test.dart
@@ -165,13 +165,11 @@ void testConfigurations(List<Map> configurations) {
       // issues with starting up a new browser just after killing the hanging
       // browser.
       maxBrowserProcesses = 1;
-    } else if (conf['runtime'].startsWith('safari') &&
-               conf['use_browser_controller']) {
+    } else if (conf['runtime'].startsWith('safari')) {
       // Safari does not allow us to run from a fresh profile, so we can only
       // use one browser.
       maxBrowserProcesses = 1;
     } else if (conf['runtime'] == 'chrome' &&
-               conf['use_browser_controller'] &&
                Platform.operatingSystem == 'macos') {
       // Chrome on mac results in random timeouts.
       maxBrowserProcesses = math.max(1, maxBrowserProcesses ~/ 2);