Add WebView Multiprocess startup benchmark.

This benchmark is almost identical to the existing WebView startup
benchmark, but with the --webview-sandboxed-renderer flag set.

Setting the pageset_repeat option for the WebView start-up benchmarks.
May make metric more stable to smooth out WebView loading from a cold
start vs. WebView being already loaded into Android's memory.

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.perf:android_s5_perf_cq;master.tryserver.chromium.perf:linux_perf_cq;master.tryserver.chromium.perf:mac_retina_perf_cq;master.tryserver.chromium.perf:winx64_10_perf_cq

Review-Url: https://codereview.chromium.org/2307873003
Cr-Commit-Position: refs/heads/master@{#417378}

tools/perf/benchmarks/system_health.py

@@ -151,7 +151,10 @@ class WebviewStartupSystemHealthBenchmark(perf_benchmark.PerfBenchmark):
   markers recorded in atrace, Chrome tracing is not enabled for this
   benchmark.
   """
-  page_set = page_sets.BlankPageSet
+  options = {'pageset_repeat': 20}
+
+  def CreateStorySet(self, options):
+    return page_sets.SystemHealthStorySet(platform='mobile', case='blank')
 
   def CreateTimelineBasedMeasurementOptions(self):
     options = timeline_based_measurement.Options()
@@ -168,3 +171,21 @@ class WebviewStartupSystemHealthBenchmark(perf_benchmark.PerfBenchmark):
   @classmethod
   def Name(cls):
     return 'system_health.webview_startup'
+
+
+@benchmark.Enabled('android-webview')
+class WebviewMultiprocessStartupSystemHealthBenchmark(
+    WebviewStartupSystemHealthBenchmark):
+  """Webview multiprocess startup time benchmark
+
+  Benchmark that measures how long WebView takes to start up
+  and load a blank page with multiprocess enabled.
+  """
+
+  def SetExtraBrowserOptions(self, options):
+    options.AppendExtraBrowserArgs(
+        ['--webview-sandboxed-renderer'])
+
+  @classmethod
+  def Name(cls):
+    return 'system_health.webview_startup_multiprocess'

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")