Settings WebUI: Add JS type checking for crostini subpage.

Also fix existing type checking violations.

Test: ninja -C out/<out_folder>/ chrome/browser/resources/settings/crostini_page:closure_compile
Bug: None
Change-Id: Ibaa65c37dec2e06a012e4767f572386134fea94b
Reviewed-on: https://chromium-review.googlesource.com/c/1423699
Reviewed-by: Jeremy Klein <jlklein@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624436}
3 files changed