Bottom toolbar button slot

This CL creates a bottom toolbar button slot that is used for the
first two buttons in the bottom toolbar (home/clear tab and search
accelerator). The

Change-Id: I6442d22498959c80b5cde8125550feb98c742dd9
Reviewed-on: https://chromium-review.googlesource.com/1139034
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Pedro Amaral <amaralp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578348}
13 files changed