Restore depot_tools autoupdate in Windows.

Original CL: https://codereview.chromium.org/1242013003/

Executing a bat file from another bat file will replace
that first bat file with the second one. To call the
second one and then return and execute the rest of the
first one, you have to do "call second.bat"

"python" is really "python.bat" so that this happened
was not obvious. More obvious now.

R=bratell@opera.com
TBR=pgervais@chromium.org
BUG=511926

Review URL: https://codereview.chromium.org/1239263003 .

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296063 0039d316-1c4b-4281-b951-d872f2087c98

update_depot_tools.bat

@@ -29,7 +29,7 @@ set errorlevel=
 
 :: Initialize/update virtualenv.
 cd /d "%DEPOT_TOOLS_DIR%."
-python -u bootstrap\bootstrap.py --deps_file bootstrap\deps.pyl --quiet ENV
+call python.bat -u bootstrap\bootstrap.py --deps_file bootstrap\deps.pyl --quiet ENV
 if errorlevel 1 goto :EOF
 
 :: Shall skip automatic update?