Needed to resolve "UnicodeDecodeError" for Python 3 presubmit checks.

Bug: 1210746
Change-Id: If55190ffc9f99c671a84fe9b3539317e464639cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2904927
Commit-Queue: Dirk Pranke <dpranke@google.com>
Auto-Submit: Alan Screen <awscreen@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>