In python3, dict.keys() returns a view instead of a separate
dictionary, which results in the following error while trying
to use cpplint:

"RuntimeError: dictionary changed size during iteration"

Use a list of the keyset instead.

R=ehmaldonado@chromium.org

Bug: 1027160
Change-Id: I0040b8912d7e446d35d755bb021670c9f2248f4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1966743Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Sarthak Kukreti <sarthakkukreti@chromium.org>