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The button defaults to FocusBehavior::ACCESSIBLE_ONLY which (apart from
on Mac) only affects buttons in AccessiblePaneViews (which the close
button is not). Make it FocusBehavior::NEVER so the behaviour is
consistent across platforms.

Note this does not affect screen readers' ability to focus the element.
Keyboard access to this element when not using a screen reader is done
via the ESC key handler in DialogClientView.

BUG=741251

Review-Url: https://codereview.chromium.org/2982533002
Cr-Commit-Position: refs/heads/master@{#486228}