Revert of Removed virtualenv from depot_tools

Revert of Removed virtualenv from depot_tools (patchset #1 id:1 of https://codereview.chromium.org/1437483002/ )

Reason for revert:
This CL over-aggressively removed the Windows bootstrap code used by /gclient.bat, which in turn downloads/installs the Windows toolchain, including Python. Labs can no longer provision Windows bots because of this.

update_depot_tools.bat did more than initialize the virtualenv. It also downloaded and installed Python, Git, and Subversion on Windows systems.

I'm reverting to restore the toolchain so we can create new Windows bots. 

Original issue's description:
> Removed virtualenv from depot_tools
> 
> This effectively reverts http://crrev.com/1195423002 and
> http://crrev.com/1205873002.
> 
> R=pgervais@chromium.org, tandrii@chromium.org
> TBR=pgervais@chromium.org  # i wanna get my Fixit credit today :-)
> BUG=542922, 503067
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=297491

TBR=pgervais@chromium.org,tandrii@chromium.org,sergiyb@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=542922, 503067
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=297493

[dnj, 2015-11-09 18:30:00]

Created Revert of Removed virtualenv from depot_tools

[dnj, 2015-11-09 18:49:46]

Committed patchset #1 (id:1) manually as 297493 (presubmit successful).

[tandrii(chromium), 2015-11-09 18:52:01]

lgtm, thanks, Dan!