Flags UI: Update to JS modules

Started from chromium-review.googlesource.com/c/chromium/src/+/1940724
by dpapad@

Bug: 1173575
Change-Id: I8343cd5990cca989033fab4d332d606c28526ba8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2819539


Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#871962}