It was previously stored in each repository working dir, making it
neccessary to modify .gitignore files in many places. Save it in
depot_tools working directory, where there's already an entry in its
.gitignore file and no additional work is needed to hide this file.

Bug: None
Change-Id: I58102180e37358a206c8ecd1044aa772a684e066
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1944167
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>