Get rid of VisibleSelection::selectionFromContentsOfNode()
This patch gets rid of |VisibleSelection::selectionFromContentsOfNode()| with
introducing |SelectionTemplate<T>::selectAllChildren()| by using
|setSelection()| taking |SelectionTemplate<T>| to simplify |VisibleSelection|
class for improving code health.
BUG=
657237
TEST=n/a; no behavior changes
Committed:
https://crrev.com/95dd59cb1bf42824ce20e77c13b51b5d33dabef0
Cr-Commit-Position: refs/heads/master@{#426437}