Don't show top toolbar items when bottom toolbar is enabled

The original CL for this (https://chromium-review.googlesource.com/1105569)
was reverted because of a null pointer bug. This CL does the same thing
as the reverted CL and fixes the null pointer exception by having
ToolbarManager#getMenuButton() return the bottom toolbar menu button when
the bottom toolbar is enabled.

Bug: 852117

Change-Id: Ic2668a3cac6a552bee4476d1cfb58f8e6c290bfd
Reviewed-on: https://chromium-review.googlesource.com/1135776
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Pedro Amaral <amaralp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575372}
5 files changed