[omnibox] Add suggestion button row for actionable items

This CL creates a button row to present actionable items including
keyword, tab switch, and pedals. No focus or keyboard handling
is considered yet, but the buttons can appear and be clicked.

Bug: 1046523
Change-Id: Iff25e0cf83ef711fbff42fe7295a81d91c4f3d98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2030044
Commit-Queue: Orin Jaworski <orinj@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737136}