Reland of lemetry: Fix startup benchmarks so that all metrics get recorded.

tools/perf/page_sets/blank_page.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.
 from telemetry.page import page as page_module
 from telemetry import story
 
 
 class BlankPage(page_module.Page):
   def __init__(self, url, page_set):
     super(BlankPage, self).__init__(url, page_set=page_set)
 
+  def RunPageInteractions(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(
+        """
+        this.hasRunRAF = 0;
+        requestAnimationFrame(function() {
+            this.hasRunRAF = 1;
+        });
+        """
+    )
+    action_runner.WaitForJavaScriptCondition("this.hasRunRAF == 1")
 
 class BlankPageSet(story.StorySet):
   """A single blank page."""
 
   def __init__(self):
     super(BlankPageSet, self).__init__()
     self.AddStory(BlankPage('file://blank_page/blank_page.html', self))