Convert remaining non-CrOS Closure Compiles to GN.

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/1049209
Reviewed-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}

• BUILD.gn[diff]
• chrome/browser/resources/BUILD.gn[diff]
• chrome/browser/resources/ntp4/BUILD.gn[Added - diff]
• chrome/browser/resources/ntp4/apps_page.js[diff]
• chrome/browser/resources/ntp4/compiled_resources2.gyp[Deleted - diff]
• components/chrome_apps/webstore_widget/externs/compiled_resources2.gyp[Deleted - diff]
• components/offline_pages/resources/BUILD.gn[Added - diff]
• components/offline_pages/resources/compiled_resources2.gyp[Deleted - diff]
• components/offline_pages/resources/renovations.js[diff]
• third_party/closure_compiler/compiled_resources2.gyp[diff]