Switch windbg_common.bat to python3

tools\win\windbg64.bat and tools\win\windbg32.bat can be used to launch
the version of Windbg packaged in the Chromium toolchain used by Google
employees, however the batch file was invoking python instead of
python3. This corrects that.

The change is necessary because build\vs_toolchain.py ends up being
invoked and this script is no longer Python 2 compatible.

Change-Id: I24f63e9bac4e4b990c78bdd602ea6991b9dd1e89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4236516
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/main@{#1103379}
NOKEYCHECK=True
GitOrigin-RevId: 19b4c1ae0cdd77b556d3af228f572e27f559bcae
1 file changed