swarming: Fix fs.islink() on Windows
swarming: Fix fs.islink() on Windows

It would incorrectly report True on missing file.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug: 755746
Change-Id: I3ece669e712944d97d01c6a4f8df0ce620fc0263
Reviewed-on: https://chromium-review.googlesource.com/636147
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py