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With https://crrev.com/c/2418443, the files using the sources
assignment filter setting explicitly set the value. The global
initialisation from build/config/BUILDCONFIG.gn is no longer
needed.

Bug: 1018739
Change-Id: Ic5517b196213b7b3f14c902f57c8716c8e1beea3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426564
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810642}