telemetry: Allow page tests to be configured via constructor

tools/perf/measurements/screenshot.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.
 import os
 
 from telemetry.page import page_test
 from telemetry.page import page_test
 from telemetry.value import scalar
 
 
 class Screenshot(page_test.PageTest):
   def __init__(self):
     super(Screenshot, self).__init__(
         action_name_to_run = 'RunPageInteractions',
         is_action_name_to_run_optional=True)
+    self._png_outdir = None
 
-  @classmethod
-  def AddCommandLineArgs(cls, parser):
+  def AddCommandLineArgs(self, parser):
     parser.add_option('--png-outdir',
                       help='Output directory for the PNG files')
 
-  @classmethod
-  def ProcessCommandLineArgs(cls, parser, args):
+  def ProcessCommandLineArgs(self, parser, args):
     if not args.png_outdir:
       parser.error('Please specify --png-outdir')
-    cls._png_outdir = args.png_outdir
+    self._png_outdir = args.png_outdir
 
   def ValidateAndMeasurePage(self, page, tab, results):
     if not tab.screenshot_supported:
       raise page_test.TestNotSupportedOnPlatformFailure(
           'Browser does not support screenshotting')
 
     tab.WaitForDocumentReadyStateToBeComplete()
     screenshot = tab.Screenshot(60)
 
     outpath = os.path.abspath(
         os.path.join(self._png_outdir, page.file_safe_name)) + '.png'
 
     if os.path.exists(outpath):
       previous_mtime = os.path.getmtime(outpath)
     else:
       previous_mtime = -1
 
     screenshot.WritePngFile(outpath)
 
     saved_picture_count = 0
     if os.path.exists(outpath) and os.path.getmtime(outpath) > previous_mtime:
       saved_picture_count = 1
     results.AddValue(scalar.ScalarValue(
         results.current_page, 'saved_picture_count', 'count',
         saved_picture_count))