[tools/perf] Remove clear_caches=False option in possible_browser.Create

This is the default behavior after:
https://chromium-review.googlesource.com/c/catapult/+/1421398

No need to specify this option any more.

Bug: 811244
Change-Id: Ia0d752bba86481ffa5e77d263a079a2f56a4da19
Reviewed-on: https://chromium-review.googlesource.com/c/1421191Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624961}

tools/perf/page_sets/dual_browser_story.py

@@ -151,7 +151,7 @@ class MultiBrowserSharedState(story_module.SharedState):
       possible_browser, browser_options = self._possible_browsers[browser_type]
       possible_browser.SetUpEnvironment(browser_options)
       possible_browser.FlushOsPageCaches()
-      self._browsers[browser_type] = possible_browser.Create(clear_caches=False)
+      self._browsers[browser_type] = possible_browser.Create()
     self._browsers_created = True
 
   def _CloseAllBrowsers(self):