Implemented telemetry benchmark that loads page and outputs screenshot.

tools/perf/contrib/cluster_telemetry/screenshot.py

+# Copyright 2017 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.
+
+import logging
+import os
+import py_utils
+import time
+
+from telemetry.page import legacy_page_test
+from telemetry.util import image_util
+
+class Screenshot(legacy_page_test.LegacyPageTest):
+  """Takes a PNG screenshot of the page."""
+
+  def __init__(self, png_outdir, wait_time=0):
+    super(Screenshot, self).__init__()
+    self._png_outdir = png_outdir
+    self._wait_time = wait_time
+
+  def ValidateAndMeasurePage(self, page, tab, results):
+    if not tab.screenshot_supported:
+      raise legacy_page_test.MeasurementFailure(
+        'Screenshotting not supported on this platform')
+
+    try:
+      tab.WaitForDocumentReadyStateToBeComplete()
+    except py_utils.TimeoutException:
+      logging.info("WaitForDocumentReadyStateToBeComplete() timeout, page: %s",
+                    page.display_name)
+      return
+
+    time.sleep(self._wait_time)
+
+    if not os.path.exists(self._png_outdir):
+      logging.info("Creating directory %s", self._png_outdir)
+      try:
+        os.makedirs(self._png_outdir)
+      except OSError:
+        logging.info("Directory %s could not be created", self._png_outdir)
+        raise
+
+    outpath = os.path.abspath(
+        os.path.join(self._png_outdir, page.file_safe_name)) + '.png'
+    # Replace win32 path separator char '\' with '\\'.
+    outpath = outpath.replace('\\', '\\\\')
+
+    screenshot = tab.Screenshot()
+
+    # TODO(lchoi): Add logging to image_util.py and/or augment error handling of
+    # image_util.WritePngFile
+    logging.info("Writing PNG file to %s", outpath)

[wkorman, 2017/06/08 23:14:41]

For nednguyen@: I asked Lawrence to add this and the logging below in an earlier
patch set. The thought process was to ensure that if we run this with -v we get
some useful output. Writing a png is an expensive operation and could take a
while. We could see a hang in there. Normally info logging probably isn't even
outputted AFAIK (if it is, switching these to debug log level would be fine).

The thought was to be kind to our future selves by adding some useful debug
logging in case it's helpful later.

I just wanted to explain. If you still think this isn't useful given you are
much more familiar with how these scripts are run, what issues crop up, etc. we
can certainly remove it.

[nednguyen, 2017/06/09 00:08:22]

I see. If this take long time when you run it locally, then make sense to
include it. Maybe making the log more explicit?

logging.info("Writing PNG file to %s. This can take a long time..", outpath)
start=time.time()
..
logging.info("PNG file written successfully (took %s seconds)",
time.time()-start)

+    image_util.WritePngFile(screenshot, outpath)
+    logging.info("PNG file written successfully")