WebUI: partial revert of migration from var to let/const

crrev.com/c/1395297 broke ARC++ opt-in dialog. Partially revert it to
unblock ARC++ development. Add comment to disable ESLint check at
original commiters request.

BUG=792774
BUG=b:122692988
TEST=Ensure arc opt-in dialog can be opened
TBR=dpapad@chromium.org

Change-Id: Iad64d4f3965dfb73097d0a806231dc4ee1fc5e5b
Reviewed-on: https://chromium-review.googlesource.com/c/1406242
Reviewed-by: Christopher Morin <cmtm@google.com>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Christopher Morin <cmtm@google.com>
Cr-Commit-Position: refs/heads/master@{#621935}
1 file changed