Patch uploaded.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e7e44dffeb2b6045cb9fe987d2a3334bf489cf39

commit e7e44dffeb2b6045cb9fe987d2a3334bf489cf39
Author: Nektarios Paisios <nektar@chromium.org>
Date: Thu Feb 22 14:13:07 2018

Only adds an ax description if it is different from the name and always exposes an ax description for Back and Forward buttons

1. In Views, tooltips were used for the accessible description attribute if the view didn't provide a description itself. However, we didn't check if the tooltip was the same as the accessible name with the effect that NVDA would sound as if it was repeating itself when the tooltip matched the accessible name.
2. For the Back and Forward buttons, an accessible description was never set. Instead, the tooltips of both buttons were modified if the browser was in accessibility mode. This is erronious because an AT might be loaded after the toolbar buttons have been initialized and because it would be strange for the tooltips to be different if an AT is running.

R=dmazzoni@chromium.org

Bug:  792704 ,  792691 
Change-Id: I2ca1fde9cfa3787ef8cb8a3a9069f74abf16dfb5
Tested: Using NVDA and manually navigating around the toolbar
Reviewed-on: https://chromium-review.googlesource.com/918081
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538426}
[modify] https://crrev.com/e7e44dffeb2b6045cb9fe987d2a3334bf489cf39/chrome/app/generated_resources.grd
[modify] https://crrev.com/e7e44dffeb2b6045cb9fe987d2a3334bf489cf39/chrome/browser/ui/cocoa/toolbar/toolbar_controller.mm
[modify] https://crrev.com/e7e44dffeb2b6045cb9fe987d2a3334bf489cf39/chrome/browser/ui/views/toolbar/toolbar_view.cc
[modify] https://crrev.com/e7e44dffeb2b6045cb9fe987d2a3334bf489cf39/tools/gritsettings/startup_resources_mac.txt
[modify] https://crrev.com/e7e44dffeb2b6045cb9fe987d2a3334bf489cf39/ui/accessibility/ax_node_data.cc
[modify] https://crrev.com/e7e44dffeb2b6045cb9fe987d2a3334bf489cf39/ui/accessibility/ax_node_data.h
[modify] https://crrev.com/e7e44dffeb2b6045cb9fe987d2a3334bf489cf39/ui/views/accessibility/view_accessibility.cc
[modify] https://crrev.com/e7e44dffeb2b6045cb9fe987d2a3334bf489cf39/ui/views/widget/native_widget_mac_accessibility_unittest.mm