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.value import scalar
+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, disable_javascript=False):
+    super(Screenshot, self).__init__()
+    self._png_outdir = png_outdir
+    self._wait_time = wait_time
+    # Javascript flag not yet implemented

[nednguyen, 2017/06/08 20:18:26]

I suggest just leave this part out until someone implement them instead of
commenting.

+    # self._disable_javascript = disable_javascript
+
+  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)

[nednguyen, 2017/06/08 20:18:26]

if this timeout, this should return instead of keep going

+
+    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('\\', '\\\\')
+
+    logging.info("Taking screenshot of %s", page.display_name)

[nednguyen, 2017/06/08 20:18:26]

this log doesn't seem particularly useful IMO

+    screenshot = tab.Screenshot()
+
+    last_mod_time = 0
+    if os.path.exists(outpath):
+        last_mod_time = os.path.getmtime(outpath)
+
+    # 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)
+    image_util.WritePngFile(screenshot, outpath)
+    logging.info("PNG file written successfully")
+
+    saved_picture_count = 0
+    if os.path.exists(outpath) and os.path.getmtime(outpath) > last_mod_time:

[nednguyen, 2017/06/08 20:18:25]

Why we need this logic? If the pgn file are not written succesfully, it should
throw exception & we don't even get to here.

[lchoi, 2017/06/08 20:38:34]

I see. Do you want me to remove this and everything that relates to mtime then?

[nednguyen, 2017/06/08 21:39:26]

sgtm

+        saved_picture_count = 1
+    results.AddValue(scalar.ScalarValue(
+            results.current_page, 'saved_picture_count', 'count',

[nednguyen, 2017/06/08 20:18:26]

is this saved_picture_count value used by Cluster Telemetry in anyway?

[lchoi, 2017/06/08 20:38:34]

Not sure if it will be used by CT - I was just following the example set by
skpicture_printer. Should I remove for now?

[nednguyen, 2017/06/08 21:39:26]

Yeah. Code is liability, so it's usually better to not add them until we have
concrete need :-)

+            saved_picture_count))
+

[nednguyen, 2017/06/08 20:18:26]

style nits: remove 3 blank lines

+
+