Fix target_os="chromeos" check in MB on CrOS bots.
Fix target_os="chromeos" check in MB on CrOS bots.

The cros wrapper actually passes 'target_os = "chromeos"' and
the check couldn't deal with spaces in the args.

TBR=stevenjb@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1916183005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#389988}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk