WebUI: Convert remaining usages of var to let/const, part 3/3.

Conversions performed with a tool (lebab). In a few occasions had to
manually convert, where the tool could not determine if the conversion
was safe.

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

Bug: 792774
Change-Id: I6df38ac00c27a412108ed841660bd758f5db0d67
Reviewed-on: https://chromium-review.googlesource.com/c/1361795
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614050}
10 files changed