[fuchsia][gn] Add os variable for fuchsia and add to default build set

Change-Id: I8ded09baf6ff050b088dd5c93b3a206a0bd6b695
Reviewed-on: https://chromium-review.googlesource.com/682475
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: James Robinson <jamesr@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#504177}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: be8980d42986c3c8b8727e761c9bc188dc2a1ffb
1 file changed
tree: 65d69fccf652bca99ac6af471c83310aa2fd88d3
  1. tools/