Use $root:default as a "default" rule

When the root build file is changed in .gn file, we should check
$root:default rather than //:default when looking for "default" rule.

Review-Url: https://codereview.chromium.org/2824153002
Cr-Original-Commit-Position: refs/heads/master@{#465758}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ddb3babce43a642e5f4e985e2a69c741057aad3c
5 files changed
tree: 39552ac3ff18f714e8b3c5bb756c677c4741f183
  1. tools/