Experimenting delaying scroll start on wikipedia page in key_mobile_sites_smooth

Currently smoothness action (e.g. scrolling) starts when the
document.readyState = interactive or complete. It is noticed that when
the readyState = interactive, the page can still be loading resources
so there would be contention on the main thread between resources loading
and input handling, which could have impact on the smoothness metrics.

It is to our interests in actually solving the contention issues so we
don't want to artificially delay the action start just to avoid the
contention. But I do see the benefit in running an experiment on just
wikipedia with delayed action start so as to understand to which degree
this contention is affecting our metrics.

So this CL makes a duplicate of the Wikipedia page in the
key_mobile_sites_smooth and running that particular page with
starting scrolling on readyState = complete.

BUG=455277

Review URL: https://codereview.chromium.org/901563003

Cr-Commit-Position: refs/heads/master@{#314932}

tools/perf/page_sets/key_mobile_sites_smooth.py

@@ -24,14 +24,19 @@ def _CreatePageClassWithSmoothInteractions(page_cls):
 
 class KeyMobileSitesSmoothPage(page_module.Page):
 
-  def __init__(self, url, page_set, name='', labels=None):
+  def __init__(self, url, page_set, name='', labels=None,
+               action_on_load_complete=False):
     super(KeyMobileSitesSmoothPage, self).__init__(
         url=url, page_set=page_set, name=name,
         credentials_path='data/credentials.json', labels=labels)
     self.user_agent_type = 'mobile'
     self.archive_data_file = 'data/key_mobile_sites.json'
+    self.action_on_load_complete = action_on_load_complete
 
   def RunPageInteractions(self, action_runner):
+    if self.action_on_load_complete:
+        action_runner.WaitForJavaScriptCondition(
+            'document.readyState == "complete"', 30)
     _IssueMarkerAndScroll(action_runner)
 
 
@@ -166,12 +171,18 @@ class KeyMobileSitesSmoothPageSet(page_set_module.PageSet):
       page_set=self,
       name='Wordpress'))
 
-   # Why: #6 (Alexa) most visited worldwide, picked an interesting page
+    # Why: #6 (Alexa) most visited worldwide, picked an interesting page
     self.AddUserStory(KeyMobileSitesSmoothPage(
       url='http://en.wikipedia.org/wiki/Wikipedia',
       page_set=self,
       name='Wikipedia (1 tab)'))
 
+    # Why: Wikipedia page with a delayed scroll start
+    self.AddUserStory(KeyMobileSitesSmoothPage(
+      url='http://en.wikipedia.org/wiki/Wikipedia',
+      page_set=self,
+      name='Wikipedia (1 tab) - delayed scroll start',
+      action_on_load_complete=True))
 
     # Why: #8 (Alexa global), picked an interesting page
     # Forbidden (Rate Limit Exceeded)