Move VisibleSelection::positionRespectingEditingBoundary() to EditingUtilities.cpp

This patch moves a member function |positionRespectingEditingBoundary()| in
|VisibleSelection| and gets rid of |visiblePositionRespectingEditingBoundary()|
to simplify |VisibleSelection| class for improving code health, since
these functions are used in one place.

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

Review-Url: https://codereview.chromium.org/2022723002
Cr-Commit-Position: refs/heads/master@{#396779}
6 files changed