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This is used for unit tests in ui/file_manager, but it is neat and
other targets should use it too. It's now pretty stable. It reduces
boilerplate in BUILD.gn files and allows foo_unittest.html files to
be automatically generated from build dependencies, rather than being
handcrafted.

js_unit_test.py currently lacks the performance optimization added
for js_binary in r581961 to cache dependency subtrees, so it's a
bit slow. Upstreaming allows the two python scripts to share the
dependency walker in a sane manner.

Bug: 860355
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Iedddd39907137dd40ec21f507cb560756251c8b1
Reviewed-on: https://chromium-review.googlesource.com/1212364
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589840}