WebUI: Convert remaining usages of var to let/const, part 2.

Conversions performed with a combination of lebab and ESLint --fix.
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: Ib0fe3108b1b6ec7b52b5f066358cf2fbae17895e
Reviewed-on: https://chromium-review.googlesource.com/c/1359465
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613645}
32 files changed