This changes autoninja.bat so that it refers explicitly to the
depot_tools version of Python rather than depending on what is in the
path. This helps to make autoninja behave more consistently and makes it
easier for us to remove python.bat from the path.

Bug: 777069
Change-Id: I6d878f58e2f8be70d06a182d059caa9fbedaedbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2753458Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>