GitLab

- In support of loading/adding/deleting a vector of icons with callbacks.
- Change Add API, removing key argument.
- Will follow-up unit test in next cl.

Bug: 963290
Change-Id: Ie0fddeea5a1dc74103fb5765a976bb3ad933e156
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1722407
Commit-Queue: Hesen Zhang <hesen@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682107}