Add stats collection for telemetry startup.warm.blank_page test.

mojo/services/tracing/public/cpp/switches.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "mojo/services/tracing/public/cpp/switches.h"
+
+namespace tracing {
+
+// Specifies if the |StatsCollectionController| needs to be bound in html pages.
+// This binding happens on per-frame basis and hence can potentially be a
+// performance bottleneck. One should only enable it when running a test that
+// needs to access the provided statistics.
+const char kEnableStatsCollectionBindings[] =
+    "enable-stats-collection-bindings";
+
+}  // namespace tracing