Remove startup_url from PageSet, we can now only specify startup_url on a Page.

tools/perf/page_sets/startup_pages.py

Index: tools/perf/page_sets/startup_pages.py
diff --git a/tools/perf/page_sets/startup_pages.py b/tools/perf/page_sets/startup_pages.py
index 2adb7cac20e01d85a6f3eeb66e232e1a17899c95..0fbfacbc821f93936183c747acdd764afb27f838 100644
--- a/tools/perf/page_sets/startup_pages.py
+++ b/tools/perf/page_sets/startup_pages.py
@@ -8,9 +8,9 @@ from telemetry.page import page_set as page_set_module
 class StartedPage(page_module.Page):
 
   def __init__(self, url, startup_url, page_set):
-    super(StartedPage, self).__init__(url=url, page_set=page_set)
+    super(StartedPage, self).__init__(
+        url=url, page_set=page_set, startup_url=startup_url)
     self.archive_data_file = 'data/startup_pages.json'
-    self.startup_url = startup_url
 
   def RunNavigateSteps(self, action_runner):
     action_runner.Wait(10)