Add WebView Multiprocess startup benchmark.

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 _BlankStory(system_health_story.SystemHealthStory):
+  """Abstract base class for blank page stories."""
+  ABSTRACT_STORY = True
+
+  def _DidLoadDocument(self, action_runner):

[mikecase (-- gone --), 2016/09/07 20:40:40]

Not 100% sure I need this?

For context, this is run for the BlankPageSet and was added in this CL

https://codereview.chromium.org/1475773003

+    # Request a RAF and wait for it to be processed to ensure that the metric
+    # Startup.FirstWebContents.NonEmptyPaint2 is recorded.
+    action_runner.ExecuteJavaScript(
+        """
+        this.hasRunRAF = 0;
+        requestAnimationFrame(function() {
+            this.hasRunRAF = 1;
+        });
+        """
+    )
+    action_runner.WaitForJavaScriptCondition("this.hasRunRAF == 1")
+
+class BlankStoryAboutBlank(_BlankStory):
+  NAME = 'blank:about:blank'
+  URL = 'about:blank'