WebUI: Check in Polymer 3 code for cr-tabs.

Bug: 1184053
Change-Id: I18655f31509e6e47435f0c3a8503843bbe2cc0b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2775468


Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#865072}