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Currently, when a label is referenced by a control of type radio or
checkbox, we set the label as ignored in the AXTree thereby excluding
it from being perceived by ATs.
However, for the case when the control (radio/checkbox) is a
descendant of the label and label is focusable (tabindex="0") but
control is to be omitted from focus (tabindex="-1"), ATs perceive
neither the label nor the control when tabbing through the content:

    <label style="display:block" tabindex="0">
      radio button 1
      <input type="radio" id="radio1" tabindex="-1">
    </label>

    <label style="display:block" for="radio2" tabindex="0">
      radio button 2
      <input type="radio" id="radio2" tabindex="-1">
    </label>

    <label style="display:block" for="checkbox1" tabindex="0">
      checkbox 1
      <input type="checkbox" id="checkbox1" tabindex="-1">
    </label>

    <label style="display:block" for="checkbox2" tabindex="0">
      checkbox 2
      <input type="checkbox" id="checkbox2" tabindex="-1">
    </label>

This change fixes the above scenario by exposing labels (referenced by
radio or checkbox) when they are focusable. For all other cases when
labels are referenced by radio or checkbox, we keep the existing
behavior of ignoring the label.

Note: Neither Firefox nor Edge legacy hides a label from AT when the
label is referenced by a control. On Firefox and Edge legacy, label
content will be read twice (focus on label and focus on control
respectively).
WAI-ARIA spec does not specify label should be ignored when associated
with a radio/checkbox control.

Bug: 1040210
Change-Id: Ie7df5144dd2f0d285d97e6556adb46dcf2338f25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992017
Commit-Queue: Victor Fei <vicfei@microsoft.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731820}