GitLab

Enable the .png image upload by default. Change the options drop down
menu to only have one option with hardcoded string "*.jpeg, *.jpg,
*png".

Screenshot:
https://screenshot.googleplex.com/1sd8JoBoM14.png
https://screenshot.googleplex.com/5mnsxOtRxJD.png

Bug: 864303
Change-Id: I3b041d0f4e08898fa0c6998742a8ce04065ab5ac
Reviewed-on: https://chromium-review.googlesource.com/1166027
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581360}