WebUI: Migrate ui/webui/resources folder for var to let/const.

Also update ESLint checks to enforce these.

Bug: 792774
Change-Id: I7219a999d8420d51af88e8f3a5edb46e1bc99722
Reviewed-on: https://chromium-review.googlesource.com/c/1395297
Reviewed-by: Dan Beam <dbeam@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621404}
58 files changed