GitLab

* Users can now set 2 tab model metadatas.
* If any fields are empty, the url metadata will be treated as nullopt.
* 3rd and 4th tabs will be added later.

Screenshots
https://screenshot.googleplex.com/AwV2evHm2c7QCMU
https://screenshot.googleplex.com/6q7tXZFAH3K8pSL
https://screenshot.googleplex.com/7rgRSDiU2EqKwXE
https://screenshot.googleplex.com/7mJtxtdg3GrDF6q

Bug: 1106937
Change-Id: I20872614efa1caa889267c1b01d4406255c60be1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391647
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804145}