Allow tabbing to anchors with explicit tabIndex even when tabsToLinks is disabled

With tabsToLinks setting enabled, browser focuses all links on pressing tab.
With it disabled, it does not focus any. This patch changes behavior when the
option is disabled so that links with explicit tabIndex are focusable. When
author has specified tabIndex, he most likely wants the element to be keyboard
accessible.

This option is by default enabled but only Mac exposes it in the settings so
it only affects Mac users who have changed default settings.

BUG=240678

Review URL: https://chromiumcodereview.appspot.com/15238002

git-svn-id: svn://svn.chromium.org/blink/trunk@157684 bbb929c8-8fbe-4397-9dbb-9b2b20218538