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This arg never changes among all supported CrOS device builds of Chrome.
By giving it a proper default val in those builds, we can reduce the amount
of inputs needed when building for CrOS (ie: simple chrome & chromeos-chrome
ebuild).

Bug: 937821
Change-Id: If383391938d84d4f584854ceb7c44428c3aa094b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1962796Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#724078}