win: Fix current toolchain path with old packaging
win: Fix current toolchain path with old packaging

I broke boringssl's use of depot_tools toolchain as I changed the
name of the sdk path key. This happened to work in Chrome because
src/ has fallback behaviour.

Restore the 'win8sdk' name when unpacking a current/old-style
toolchain package (i.e. what everyone is still using).

TBR=dpranke@chromium.org
R=davidben@chromium.org

Review URL: https://codereview.chromium.org/1159433006

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@295492 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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