Sorting blink_perf story sets to be in a determinisitic order

Bug: 849541
Change-Id: Idbd3c3a0339c933c70a5a2335a1faca5f507eb72
Reviewed-on: https://chromium-review.googlesource.com/1089118
Commit-Queue: Emily Hanley <eyaich@chromium.org>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#565014}

tools/perf/benchmarks/blink_perf.py

@@ -84,7 +84,7 @@ def CreateStorySetFromPath(path, skipped_file,
 
   all_urls = [p.rstrip('/') for p in page_urls]
   common_prefix = os.path.dirname(os.path.commonprefix(all_urls))
-  for url in page_urls:
+  for url in sorted(page_urls):
     name = url[len(common_prefix):].strip('/')
     ps.AddStory(_BlinkPerfPage(
         url, ps, ps.base_dir,