Update base/extractor.[cc|h] for security review.

Change-Id: I58430f3609301ec6b07610917345c958c67cc26a
Reviewed-on: https://chromium-review.googlesource.com/223041
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Jack Hou <jackhou@chromium.org>
Tested-by: Jack Hou <jackhou@chromium.org>