[run_web_tests] Remove unused pixel_tests parameter in Driver

Bug: 703899

Change-Id: I3b19b91c5e74401e7ba576112d68f0adc9893aca
Reviewed-on: https://chromium-review.googlesource.com/c/1277286
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599249}
10 files changed