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: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295492 0039d316-1c4b-4281-b951-d872f2087c98