WebUI: Convert remaining usages of var to let/const, part 1.

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: I0d5fa98d5efb935e84c1990db635f96df5957e4d
Reviewed-on: https://chromium-review.googlesource.com/c/1357726
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613328}
33 files changed