llvm-build GCS dependency needs to place one GCS object based on
platform. We can't have multiple top-level GCS llvm-build dependencies
as the pathname is used as a key.
This extends object property of GCS dependency to have `condition`.
The `condition` field will be AND with the top level condition.
R=jojwang@google.com, kimstephanie@google.com
Change-Id: I06a6281140a8057d598a6eac2b07c7577f0ea86c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5527474
Reviewed-by: Stephanie Kim <kimstephanie@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
