Support for running a limited amount of browser tests in parallel

tools/test-runtime.dart

Index: tools/test-runtime.dart
diff --git a/tools/test-runtime.dart b/tools/test-runtime.dart
index a959f23424f8305b990fb60d881cab5d610d9baf..036c861d05daecc0b2cb80766248b837b905d4a5 100755
--- a/tools/test-runtime.dart
+++ b/tools/test-runtime.dart
@@ -102,9 +102,12 @@ main() {
     if (runningBrowserTests) startHttpServer('127.0.0.1', 9876);
   }
 
+  var maxBrowserProcesses = maxProcesses;
+
   // Start process queue.
   new ProcessQueue(
       maxProcesses,
+      maxBrowserProcesses,
       progressIndicator,
       startTime,
       printTiming,