win toolchain downloader: Don't omit hidden or system files.
win toolchain downloader: Don't omit hidden or system files.

Hashes for both the 2013 and 2015 toolchain don't change for me locally with
this change, looks like there are no hidden or system files in the toolchain.
(I tried with the "you are a googler" flow.)

No intended behavior change.

BUG=495204

Review URL: https://codereview.chromium.org/1200113004

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@295804 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools