While I'm here, change to a more concise way of spelling the platform
string.

Bug: 1179513
Change-Id: Ibb9bd69fb00e66332f8af73154f4615c20efb2b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2703228
Auto-Submit: Owen Rodley <orodley@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>