[iOS] Add common files used for JS overlay UI.

Bug: 941745
Change-Id: I5740baabe69011dd57bb9a9e68b7f84fa6a0a8f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637745
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666824}