ninja windows: check for some required environment variables
ninja windows: check for some required environment variables

If these variables aren't available, some tools error out (unable to create temp files, etc.)

R=thakis@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10733002
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp