[tools] Always set --data-path for content_shell

Making --data-path a subdir of --user-data-dir makes it easier to clean up
data after a benchmark run.

Bug: chromium:861668
Change-Id: If44527163ea396b11346d65d76411d03a5b9a424
Reviewed-on: https://chromium-review.googlesource.com/c/1292065Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56856}

tools/callstats.py

@@ -140,6 +140,8 @@ def get_chrome_flags(js_flags, user_data_dir, arg_delimiter=""):
       "--no-first-run",
       "--user-data-dir={}{}{}".format(arg_delimiter, user_data_dir,
                                       arg_delimiter),
+      "--data-path={}{}{}".format(arg_delimiter,
+          os.path.join(user_data_dir, 'content-shell-data'), arg_delimiter),
     ]
 
 def get_chrome_replay_flags(args, arg_delimiter=""):