These should probably block MacViews/Harmony being on by default on Mac. Which I've been calling "Phase 3" --  Issue 603386 

I think these are used a lot on ChromeOS, but last time I looked there was very little remaining on Mac that actually called these methods..

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/165afb90bd7e9991857a9b57e6d4c18bb1d47d34

commit 165afb90bd7e9991857a9b57e6d4c18bb1d47d34
Author: Elly Fong-Jones <ellyjones@google.com>
Date: Thu Jun 29 18:43:28 2017

MacViews: wire SimpleMessageBox functions

These functions are used in a few error paths throughout the browser. They are
not easy to summon on demand; the approach that I took to test this was to hack
a call to each function into [BrowserWindowController addFindBar:] and hit
command-f.

This change:
1) Renames the Views ShowMessageBoxImpl to ShowMessageBoxViews and exposes it
2) Renames the Cocoa ShowMessageBox to ShowMessageBoxCocoa and exposes it
3) Adds a bridge that invokes one variant or the other depending on the value
   of --secondary-ui-md
4) Changes the order of ShowMessageBoxCocoa's arguments to match that of
   ShowMessageBoxViews

BUG= 728157 

Change-Id: Ib731e40f63cecc50d2a9e2214cd54d21af355f70
Reviewed-on: https://chromium-review.googlesource.com/535813
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#483437}
[modify] https://crrev.com/165afb90bd7e9991857a9b57e6d4c18bb1d47d34/chrome/browser/ui/BUILD.gn
[add] https://crrev.com/165afb90bd7e9991857a9b57e6d4c18bb1d47d34/chrome/browser/ui/cocoa/simple_message_box_bridge_views.mm
[add] https://crrev.com/165afb90bd7e9991857a9b57e6d4c18bb1d47d34/chrome/browser/ui/cocoa/simple_message_box_cocoa.h
[add] https://crrev.com/165afb90bd7e9991857a9b57e6d4c18bb1d47d34/chrome/browser/ui/cocoa/simple_message_box_cocoa.mm
[delete] https://crrev.com/dca98f635b5337b08df7a3d9480800adfeaba4c5/chrome/browser/ui/cocoa/simple_message_box_mac.mm
[modify] https://crrev.com/165afb90bd7e9991857a9b57e6d4c18bb1d47d34/chrome/browser/ui/simple_message_box.h
[modify] https://crrev.com/165afb90bd7e9991857a9b57e6d4c18bb1d47d34/chrome/browser/ui/views/simple_message_box_views.cc
[add] https://crrev.com/165afb90bd7e9991857a9b57e6d4c18bb1d47d34/chrome/browser/ui/views/simple_message_box_views.h

This is fixed now. This path is used for the "open many tabs" warning, and for some other obscure error paths on Mac.