Fix gn CHECK()ing on not_needed() when given undefined identifiers

GN now gives an error message that looks like:

ERROR at //BUILD.gn:76:23: Undefined identifier
  not_needed(["deps", "deps_____________"])
                      ^------------------

R=dpranke

Change-Id: Ic55afef672a7aefab7934875ceb22aff52b68f94
Reviewed-on: https://chromium-review.googlesource.com/1063118
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#559899}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4497a44f0d8236cb069ed89c013154c1adfc2087
1 file changed
tree: d710681602dab0518b11ef24b8334eb49f31bc89
  1. tools/