Move VisibleSelection::nonBoundaryShadowTreeRootNode() to FrameSelection.cpp

This patch moves a member function |nonBoundaryShadowTreeRootNode()| in
|VisibleSelection| to "FrameSelection.cpp", where only the place uses it,
to simplify |VisibleSelection| class for improving code health.

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

Review-Url: https://codereview.chromium.org/2023793002
Cr-Commit-Position: refs/heads/master@{#396783}
3 files changed