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
Committed:
https://crrev.com/02a3e9549a9be351eb995a6e69ef949d60674900
Cr-Commit-Position: refs/heads/master@{#427633}