WebUI: Use const/let instead of var in cr_components/.

Conversions performed with a tool
./node_modules/lebab/bin/index.js \
    --replace ui/webui/resources/cr_components --transform let

A .eslintrc.js file already exists, so probably these violations had
not been fixed at the time it was added.

Bug: 792774
Change-Id: I647bbf9a08c816fe810acf6582a5d9a0a45a96a0
Reviewed-on: https://chromium-review.googlesource.com/c/1354575
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612822}