Fix "git cl upload --use-oauth2" not working on Windows when HOME is not set.
Fix "git cl upload --use-oauth2" not working on Windows when HOME is not set.

depot_tools' git wrapper for some reason overrides %HOME% to point to
depot_tools directory, and thus scripts running as git subprocesses (git_cl.py)
use not the same HOME as scripts that run outside of git process tree
(depot-tools-auth.py). Instead always use USERPROFILE in auth.py to consistently
pick same directory.

This issue happens only when HOME is not set globally. It can explain why it
wasn't seen earlier.

R=iannucci@chromium.org
BUG=356813

Review URL: https://codereview.chromium.org/1082833004

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@294911 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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