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This CL adds the getAriaLabel() and getAriaDescription() helper
functions to the __GCrWeb.fill "namespace". These functions return
the ARIA label and description strings associated with an element.

The ARIA Label is the text referenced by one or more containing element
ids in the labelled elements aria-labelledby attribute, or the value
of the element's aria-label attribute, with preference given to the
aria-labelledby attribute. For example:

    <div id='foo-label'>Label for Foo</div>
    <input name='foo' aria-labelledby='foo-label'>
    <input name='bar' aria-label='Label for Bar'>

The aria-description is similar to aria-labelledby. For example:

    <div id='foo-description'>Long descriptive text for Foo</div>
    <input name='foo' aria-describedby='foo-descrition'>

Bug: 896719
Change-Id: I300b84298fc20e1b86474f60d08cdd1c9569036b
Reviewed-on: https://chromium-review.googlesource.com/c/1325032Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606534}