[run-webkit-tests] Let --reset-results reset flag-specific baselines

If --additional-driver-flag is specified, it was weird to still let
--reset-results reset the existing baselines. There seems no use case
for that.

Now let --reset-results reset flag-specific baselines if --additional-
driver-flag is specified.

Bug: 660231
Change-Id: I6ee5a5f563a7f972fcece763256d7f99f3e97cbe
Reviewed-on: https://chromium-review.googlesource.com/740841
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512061}
4 files changed