Replace run_selenium.py with Dart script that uses the browser controller for performance tests.

tools/testing/dart/browser_controller.dart

Index: tools/testing/dart/browser_controller.dart
diff --git a/tools/testing/dart/browser_controller.dart b/tools/testing/dart/browser_controller.dart
index 462563d48b4d2e5929032901439213738c148e3a..a67f3a7da558a5cff30cf7b2ee9762e3f86fd61f 100644
--- a/tools/testing/dart/browser_controller.dart
+++ b/tools/testing/dart/browser_controller.dart
@@ -764,15 +764,18 @@ class BrowserTestRunner {
                     this.localIp,
                     this.browserName,
                     this.maxNumBrowsers,
-                    {bool this.checkedMode: false});
+                    {bool this.checkedMode: false,
+                    BrowserTestingServer this.testingServer});
 
   Future<bool> start() {
     // If [browserName] doesn't support opening new windows, we use new iframes
     // instead.
     bool useIframe =
         !Browser.BROWSERS_WITH_WINDOW_SUPPORT.contains(browserName);
-    testingServer = new BrowserTestingServer(
-        globalConfiguration, localIp, useIframe);
+    if (testingServer == null) {
+      testingServer = new BrowserTestingServer(
+          globalConfiguration, localIp, useIframe);
+    }
     return testingServer.start().then((_) {
       testingServer.testDoneCallBack = handleResults;
       testingServer.testStatusUpdateCallBack = handleStatusUpdate;
@@ -1159,7 +1162,7 @@ class BrowserTestingServer {
             print("Textresponse $textResponse");
             throw "Error returning content to browser: $error";
           }
-      });
+        });
       }
       void errorHandler(e) {
         if (!underTermination) print("Error occured in httpserver: $e");