WebUI: Convert remaining usages of var to let/const, part 4.

This is in preparation of turning on no-var, and prefer-const ESLint
checks for all of chrome/browser/resources/

Bug: 792774
Change-Id: Id1bfd8720970876275499854d4fccce1c911d0b2
Reviewed-on: https://chromium-review.googlesource.com/c/1364053
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614286}
6 files changed