Currently, the Windows toolchain is installed in a subdirectory under
"depot_tools". Allow this installation destination to be overridden,
either by flag (useful for testing) or via environment variable (to be
used in Chromium recipe).

BUG=chromium:727917
TEST=local
  - Ran explicitly w/ flag, seems to work.
  - Ran via Chromium's "vs_toolchain" with and without environment
    variable override, seems to install correctly.

Change-Id: I6b33832d7f099796e23da0548949073c70a17788
Reviewed-on: https://chromium-review.googlesource.com/521663Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>