Get rid of an unused function FrameSelection::setNonDirectionalSelectionIfNeeded() for VisibleSelection

This patch gets rid of an unused function |VisibleSelection| version of
|FrameSelection::setNonDirectionalSelectionIfNeeded()| to reduce source code
size for improving code health.

BUG=n/a
TEST=n/a; no behavior changes

Review-Url: https://codereview.chromium.org/2024703005
Cr-Commit-Position: refs/heads/master@{#397084}
2 files changed