[System Health] Tidy up stories for Chrome's special pages

tools/perf/page_sets/system_health/blank_stories.py

-# Copyright 2016 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.
-
-from page_sets.system_health import system_health_story
-
-
-class BlankAboutBlankStory(system_health_story.SystemHealthStory):
-  """Story that loads the about:blank page."""
-
-  NAME = 'blank:about:blank'
-  URL = 'about:blank'
-
-  def _DidLoadDocument(self, action_runner):
-    # Request a RAF and wait for it to be processed to ensure that the metric
-    # Startup.FirstWebContents.NonEmptyPaint2 is recorded.
-    action_runner.ExecuteJavaScript(
-        """
-        window.__hasRunRAF = false;
-        requestAnimationFrame(function() {
-          window.__hasRunRAF = true;
-        });
-        """
-    )
-    action_runner.WaitForJavaScriptCondition("window.__hasRunRAF")