[tools/perf] Tweak memory.top_10_mobile story flow

This change modifies the benchmark's story flow in two ways:
- about:blank will no longer be loaded before pushing the browser to
  the background. Thus making the background measurement more realistic
- the browser will now be restarted between each foreground/background
  pair of stories; this will make each pair stories independent of each
  other, helping to make results more reproducible and less noisy.

Perf sheriffs warning: This is expected to affect memory measurements
for this benchmark.

Bug: 750055
Change-Id: I65ea9431d52f63ee3e7dfccad68a62476018ea99
Reviewed-on: https://chromium-review.googlesource.com/612980
Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#501274}

tools/perf/page_sets/memory_top_10_mobile.py

@@ -14,15 +14,9 @@ from page_sets import top_10_mobile
 
 
 class Top10MobileSharedState(shared_page_state.SharedMobilePageState):
-  def __init__(self, test, finder_options, story_set):
-    super(Top10MobileSharedState, self).__init__(
-        test, finder_options, story_set)
-    self._story_set = story_set
-
   def ShouldStopBrowserAfterStoryRun(self, story):
-    # Close the browser after the last story in the set.
-    # TODO(crbug.com/750055): Switch to close after each background page.
-    return self._story_set[-1] == story
+    # Close the browser after each background story.
+    return isinstance(story, BackgroundPage)
 
 
 class MemoryMeasurementPage(page_module.Page):
@@ -58,17 +52,17 @@ class BackgroundPage(MemoryMeasurementPage):
   def __init__(self, story_set, name):
     super(BackgroundPage, self).__init__(story_set, name, 'about:blank')
 
-  def RunPageInteractions(self, action_runner):
-    action_runner.tab.WaitForDocumentReadyStateToBeComplete()
-
+  def RunNavigateSteps(self, action_runner):
     # Launch clock app, pushing Chrome to the background.
     android_browser = action_runner.tab.browser
     android_browser.Background()
 
+  def RunPageInteractions(self, action_runner):
     # Take measurement.
     action_runner.MeasureMemory(self.story_set.DETERMINISTIC_MODE)
 
     # Go back to Chrome.
+    android_browser = action_runner.tab.browser
     android_browser.platform.android_action_runner.InputKeyEvent(
         keyevent.KEYCODE_BACK)
 
@@ -86,6 +80,8 @@ class MemoryTop10Mobile(story_module.StorySet):
       # We name pages so their foreground/background counterparts are easy
       # to identify. For example 'http://google.com' becomes
       # 'http_google_com' and 'after_http_google_com' respectively.
+      # This also allows to use e.g. '--story-filter http_google_com' to
+      # match and run both parts of the story.
       name = re.sub(r'\W+', '_', url)
       self.AddStory(ForegroundPage(self, name, url))
       self.AddStory(BackgroundPage(self, 'after_' + name))