move_blink_source.py: Update _PATH_RULES_SPECIFIER in check-webkit-style.

It is a list of exceptional rules, and contains some Source/ paths.

Bug: 578345
Change-Id: I17e65d35c4809b9c91217db086e1af10d82dea54
Reviewed-on: https://chromium-review.googlesource.com/974605
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544991}