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This change introduces the GN arg 'external_devtools_frontend', which
for now defaults to false. Follow up changes will implement the true case.

Bug: 1011259
Change-Id: I07afe85f1552516a02184c430fb6e54d3928f04b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838192
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704166}