Get rid of a redundant FrameSelection::setSelection() with VisibleSelectionInFlatTree

This patch gets rid of a redundant |FrameSelection::setSelection()| with
|VisibleSelectionInFlatTree|, since there is one call site, to simplify
source code for improving code health.

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

Review-Url: https://codereview.chromium.org/2020143002
Cr-Commit-Position: refs/heads/master@{#396792}
2 files changed