Reland "Change GN to disallow reading args defined in the same..."

This re-lands r433944, "Change GN to disallow reading args defined in
the same declare_args() call." The change was reverted in r439366 when
we though it might be the cause of some flakiness, but that turned out
to be wrong (the change was fine).

TBR=brettw@chromium.org
BUG=542846

Review-Url: https://codereview.chromium.org/2630983002
Cr-Original-Commit-Position: refs/heads/master@{#443808}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c50ed7de30182bfd6d5eb36e049be98b497d13ac
7 files changed
tree: 88a582dcfdfc24fa69177aee8de4e108b6cc57db
  1. tools/