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This is a reland of e14ed878
With correct output path to v4l2/v4l2_stubs.{h,cc}

Original change's description:
> Make a GN template for generate_stubs.py
>
> Make it produce a source_set which can be depended on rather than directly including sources into the parent target.
>
> Bug: chromium:682122
> Change-Id: I7ad9f94459f187481e656e3513eabb3e2a6f92a1
> Reviewed-on: https://chromium-review.googlesource.com/c/1348042
> Reviewed-by: Dan Sanders <sandersd@chromium.org>
> Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Commit-Queue: Oleh Prypin <oprypin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#611117}

Bug: chromium:682122, chromium:909020
Change-Id: I1d34d5bb0dc9f4f72a444e9463669793c3f5a171
Reviewed-on: https://chromium-review.googlesource.com/c/1356703Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Oleh Prypin <oprypin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613300}