Use weblayerStartupMetric for system_health.weblayer_startup

The metric was split out of the WebView version, so we should use this
now for WebLayer startup tests.

Pinpoint job running:
https://pinpoint-dot-chromeperf.appspot.com/job/104b30a9520000

Bug: 1146438
Change-Id: I9938614b94740a3222fb832ab994427b136bf1d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536797Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827339}

tools/perf/benchmarks/system_health.py

@@ -249,12 +249,13 @@ class WebviewStartupSystemHealthBenchmark(perf_benchmark.PerfBenchmark):
 @benchmark.Info(emails=['cduvall@chromium.org', 'weblayer-team@chromium.org'],
                 component='Internals>WebLayer',
                 documentation_url='https://bit.ly/36XBtpn')
-class WebLayerStartupSystemHealthBenchmark(WebviewStartupSystemHealthBenchmark):
+class WebLayerStartupSystemHealthBenchmark(perf_benchmark.PerfBenchmark):
   """WebLayer startup time benchmark
 
   Benchmark that measures how long WebLayer takes to start up
   and load a blank page.
   """
+  options = {'pageset_repeat': 20}
   # TODO(rmhasan): Remove the SUPPORTED_PLATFORMS lists.
   # SUPPORTED_PLATFORMS is deprecated, please put system specifier tags
   # from expectations.config in SUPPORTED_PLATFORM_TAGS.
@@ -262,9 +263,17 @@ class WebLayerStartupSystemHealthBenchmark(WebviewStartupSystemHealthBenchmark):
   SUPPORTED_PLATFORM_TAGS = [platforms.MOBILE]
   SUPPORTED_PLATFORMS = [story.expectations.ALL_MOBILE]
 
+  def CreateStorySet(self, options):
+    return page_sets.SystemHealthBlankStorySet()
+
   def CreateCoreTimelineBasedMeasurementOptions(self):
-    options = super(WebLayerStartupSystemHealthBenchmark,
-                    self).CreateCoreTimelineBasedMeasurementOptions()
+    options = timeline_based_measurement.Options()
+    options.SetTimelineBasedMetrics(['weblayerStartupMetric'])
+    options.config.enable_atrace_trace = True
+    # TODO(crbug.com/1028882): Recording a Chrome trace at the same time as
+    # atrace causes events to stack incorrectly. Fix this by recording a
+    # system+Chrome trace via system perfetto on the device instead.
+    options.config.enable_chrome_trace = False
     options.config.atrace_config.app_name = 'org.chromium.weblayer.shell'
     return options