telemetry: Disable top sites in thread_times benchmarks.

tools/perf/benchmarks/silk_flags.py

 # Copyright 2014 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.
 
 
 def CustomizeBrowserOptionsForSoftwareRasterization(options):
   """Enables flags needed for forced software rasterization."""
   options.AppendExtraBrowserArgs('--disable-gpu-rasterization')
 
 
 def CustomizeBrowserOptionsForGpuRasterization(options):
   """Enables flags needed for forced GPU rasterization using Ganesh."""
   options.AppendExtraBrowserArgs('--enable-threaded-compositing')
   options.AppendExtraBrowserArgs('--enable-impl-side-painting')
   options.AppendExtraBrowserArgs('--force-gpu-rasterization')
 
 
 def CustomizeBrowserOptionsForSyncScrolling(options):
   """Enables flags needed for synchronous (main thread) scrolling."""
   options.AppendExtraBrowserArgs('--disable-threaded-scrolling')
+
+def CustomizeBrowserOptionsForThreadTimes(options):
+  """Disables options known to cause noise in thread times"""
+  # Remove once crbug.com/621128 is fixed.
+  options.AppendExtraBrowserArgs('--disable-top-sites')