WebUI: convert more let to const in c/r/b

Automatically convert more let to const in c/r/b using "eslint --fix".

Bug: 792774
Change-Id: If4422807022368c2c29a03cca78ce10eb6c4be87
Reviewed-on: https://chromium-review.googlesource.com/c/1385114
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618071}
38 files changed