Implemented telemetry benchmark that loads page and outputs screenshot.

tools/perf/measurements/screenshot_unittest.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 shutil
+import tempfile
+
+from telemetry.testing import options_for_unittests
+from telemetry.testing import page_test_test_case
+
+from measurements import screenshot
+
+class ScreenshotUnitTest(page_test_test_case.PageTestTestCase):
+
+  def setUp(self):
+    self._options = options_for_unittests.GetCopy()
+    self._png_outdir = tempfile.mkdtemp('_png_test')
+
+  def tearDown(self):
+    shutil.rmtree(self._png_outdir)
+
+  def testPixelDiff(self):
+    page_set = self.CreateStorySetFromFileInUnittestDataDir('blank.html')
+    measurement = screenshot.Screenshot(self._png_outdir)
+    results = self.RunMeasurement(measurement, page_set, options=self._options)
+
+    # Screenshots are not supported on all platforms
+    if results.failures:

[wkorman, 2017/06/08 17:57:53]

Looks like we still need to figure out some way to validate that we actually run
a screenshot on linux vs hitting this return case?

+      assert 'not supported' in results.failures[0].exc_info[1].message
+      return
+
+    saved_picture_count = results.FindAllPageSpecificValuesNamed(
+        'saved_picture_count')
+    self.assertEquals(len(saved_picture_count), 1)
+    self.assertGreater(saved_picture_count[0].GetRepresentativeNumber(), 0)