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See go/closure-compile-gn for details.

This CL converts ntp4 and offline_pages to GN Closure compilation. It
also deletes the webstore_widget GYP file which is no longer used.

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.
A more localized compile may look like:
ninja -C out/Default chrome/browser/resources/ntp4:closure_compile

TBR=fukino@chromium.org

Bug: 632206
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Id6f4c248eaf19a63084e1734bc987b5f8a7c11fc
Reviewed-on: https://chromium-review.googlesource.com/1049209Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559464}