telemetry: Fix startup benchmarks so that all metrics get recorded.

tools/perf/page_sets/blank_page.py

Index: tools/perf/page_sets/blank_page.py
diff --git a/tools/perf/page_sets/blank_page.py b/tools/perf/page_sets/blank_page.py
index 6f87074b3f80a86df5893196d7212671e7400a6c..92d46b01f539c1833d3121eb3874097eaa2943b3 100644
--- a/tools/perf/page_sets/blank_page.py
+++ b/tools/perf/page_sets/blank_page.py
@@ -9,6 +9,18 @@ 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.NonEmptyPaint 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."""