[layoutng] Add contrib layoutng benchmark

This just runs blink_perf.layout with the NG flag.

TESTED=tools/perf/run_benchmark run blink_perf.layout_ng --browser=release -v
(and verified that the commandline includes the layoutng flag)

Change-Id: I4302330462d52291d96d3d8509498337e267a1f4
Reviewed-on: https://chromium-review.googlesource.com/1185641Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585475}

tools/perf/contrib/blink_layoutng_perf/OWNERS

+eae@chromium.org
+cbiesinger@chromium.org

tools/perf/contrib/blink_layoutng_perf/blink_layoutng_perf.py

+# Copyright 2018 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.
+from benchmarks import blink_perf
+from telemetry import benchmark
+
+# pylint: disable=protected-access
+@benchmark.Info(emails=['cbiesinger@chromium.org'],
+                documentation_url='https://bit.ly/blink-perf-benchmarks')
+class BlinkPerfLayoutNg(blink_perf._BlinkPerfBenchmark):
+  subdir = 'layout'
+
+  def SetExtraBrowserOptions(self, options):
+    super(BlinkPerfLayoutNg, self).SetExtraBrowserOptions(options)
+    options.AppendExtraBrowserArgs('--enable-blink-features=LayoutNG')
+
+  @classmethod
+  def Name(cls):
+    return 'blink_perf.layout_ng'