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Allows each target to then append and customize the license file based
on their third-party sub-licenses.

Bug: 869935
Change-Id: I732c132abaf386f7ce763fb7452384717c1264ed
Reviewed-on: https://chromium-review.googlesource.com/1158964
Commit-Queue: Peter Wen <wnwen@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579867}