[iOS] Change toolbar button from Omnibox to NewTab

This CL changes the central button of the bottom toolbar from focusing
the omnibox to opening a new tab.
This needs to be activated through the flag toolbar-new-tab-button.

Bug: 962456
Change-Id: I8afa107ad1dc7e822eb8e899ef508857ca6c6707
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1662454
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671646}
29 files changed