Support `testonly` for closure_compile targets in ui/file_manager.

`testonly` ensures that test-only utility files are not depended on by
release files. It also helps when navigating deps in GN's tools.

To cut down on boilerplate, introduce `js_unittest` that automatically
adds testonly = true to js_library and is a better description of these
targets.

Leave "test_support" libraries using js_library + testonly.

Bug: 879035
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I5463718ef3f13ead4fd1efae150ff1f7830e43ad
Reviewed-on: https://chromium-review.googlesource.com/c/1275725
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600200}
11 files changed