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

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

Also adding .eslintrc.js file to prevent future additions of 'var' in that folder.

Bug: 792774
Change-Id: Ia7de669ffa717bc898fdc2989b9820a156ac1b1a
Reviewed-on: https://chromium-review.googlesource.com/c/1354565
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612022}
28 files changed