Get rid of createVisibleSelection() taking two Position

This patch gets rid of flat tree version of |createVisibleSelection()| taking
two |Position| by replacing with |SelectionInDOMTree| version to reduce number
of overloads for improving code health.

BUG=657237
TEST=n/a; no behavior changes

Review-Url: https://codereview.chromium.org/2451613003
Cr-Commit-Position: refs/heads/master@{#427633}
15 files changed