Only download the VS toolchain if DEPOT_TOOLS_WIN_TOOLCHAIN=1.
Only download the VS toolchain if DEPOT_TOOLS_WIN_TOOLCHAIN=1.

BUG=b/13171670
R=fbarchard@google.com, kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15409004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1152fe2247700c22fc71ea55f3ee687f71bdc5aa

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc