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gn has a function not_needed() intended to avoid "Variable is not
used" errors in gni files. So far the code has used asserts for the
same purpose but it is better to use the function intended for the
purpose.

Change-Id: I5ec7e50c4089b265e104aaede24d0e4e50452cef
Reviewed-on: https://chromium-review.googlesource.com/c/1314588Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#605338}