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With this, gn format //BUILD.gn results in no changes, other than to
make 'deps += [ "a" ]' turn into one line instead of multiline.

(Child of https://codereview.chromium.org/608593006/)

R=brettw@chromium.org
BUG=348474

Review URL: https://codereview.chromium.org/606123002

Cr-Commit-Position: refs/heads/master@{#297231}