Patti's been getting pretty familiar with this stuff.

In progress here: https://codereview.chromium.org/2578303003

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dc7d397153bcbccbc21e12f2ea3568f594699220

commit dc7d397153bcbccbc21e12f2ea3568f594699220
Author: patricialor <patricialor@chromium.org>
Date: Mon Jan 16 03:10:27 2017

a11y: Add a11y information to views::Tab and manually ignore its a11y children.

Currently, Views tabs have no a11y information specific to their class. The
fallback then is to use the views::Label inside them for a11y information
instead. This means they report their role as text. Fix this by adding a11y
information to views::Tab to report its role as ui::AX_ROLE_TAB, and handle the
ui::AX_ACTION_SET_SELECTED action and plumb through for Mac.

views::Tab is also a special case where only the selected tab is allowed to be
keyboard focusable. Because of this behavior, the views::Label inside is exposed
to the accessibility tree. Since views::Tab now has its own a11y information,
the views::Label should be hidden from users (see  Issue 610589 ). Make a class
views::TabLabel to manually set its a11y role to ui::AX_ROLE_IGNORED.

BUG= 657593 ,  610589 

Review-Url: https://codereview.chromium.org/2578303003
Cr-Commit-Position: refs/heads/master@{#443837}

[modify] https://crrev.com/dc7d397153bcbccbc21e12f2ea3568f594699220/chrome/app/generated_resources.grd
[modify] https://crrev.com/dc7d397153bcbccbc21e12f2ea3568f594699220/chrome/browser/ui/cocoa/tabs/tab_view.mm
[modify] https://crrev.com/dc7d397153bcbccbc21e12f2ea3568f594699220/ui/accessibility/platform/ax_platform_node_mac.mm
[modify] https://crrev.com/dc7d397153bcbccbc21e12f2ea3568f594699220/ui/strings/ui_strings.grd
[modify] https://crrev.com/dc7d397153bcbccbc21e12f2ea3568f594699220/ui/views/BUILD.gn
[modify] https://crrev.com/dc7d397153bcbccbc21e12f2ea3568f594699220/ui/views/controls/tabbed_pane/tabbed_pane.cc
[modify] https://crrev.com/dc7d397153bcbccbc21e12f2ea3568f594699220/ui/views/controls/tabbed_pane/tabbed_pane.h
[add] https://crrev.com/dc7d397153bcbccbc21e12f2ea3568f594699220/ui/views/controls/tabbed_pane/tabbed_pane_accessibility_mac_unittest.mm
[modify] https://crrev.com/dc7d397153bcbccbc21e12f2ea3568f594699220/ui/views/controls/tabbed_pane/tabbed_pane_unittest.cc