[tools/perf] Fix PressStory measurement collection

When running the story multiple times, we should clean up measurements
from previous runs.

Bug: 1041449
Change-Id: I722f4edb1083fd5bc3f2d22280434e44d6229a5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2000141Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Mikhail Khokhlov <khokhlov@google.com>
Cr-Commit-Position: refs/heads/master@{#731906}

tools/perf/page_sets/press_story.py

@@ -81,6 +81,10 @@ class PressStory(page_module.Page):
   def GetMeasurements(self):
     return self._measurements
 
+  def Run(self, shared_state):
+    self._measurements = []
+    super(PressStory, self).Run(shared_state)
+
   def ExecuteTest(self, action_runner):
     pass