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See go/closure-compile-gn for details.

This CL converts the Closure Compilation of the file manager from GYP to
GN, and fixes a number of type-checking issues that were somehow
suppressed due to the way the compiler used to be run.

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 ui/file_manager:closure_compile

Bug: 632206
TBR=tbreisacher@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ic95ae50ee2e61cb6b5432cb4c5e13a8ac3b2bb44
Reviewed-on: https://chromium-review.googlesource.com/1018684
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Sasha Morrissey <sashab@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553872}