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 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")