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See go/closure-compile-gn for details.

This CL converts the Closure Compilation from GYP to GN for the following:
- cryptotoken
- download_internals
- md_downloads
- md_extensions
- md_history
- md_user_manager
- offline_pages
- signin
- webapks
- welcome

A local closure compile can be run by setting closure_compile = true in
args.gn, and then running ninja -C out/Default webui_closure_compile.
Individual projects can be compiled via, for example,
ninja -C out/Default chrome/browser/resources/md_extensions:closure_compile

Bug: 632206
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I401fc54ee6c5202bd27855ba28d886677e52210e
Reviewed-on: https://chromium-review.googlesource.com/954772
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550956}