Return null SelectionPaintRange if no LayoutObject.

There is a case that CreateVisiblePosition returns {start, null} range
 in LayoutSelection::CalcSelection.
VisibleSelection and EphemeralRange constructors prohibit such case
 but it is by DCHECK so we need cover on Release build failure.

BUG=739996

Review-Url: https://codereview.chromium.org/2968363002
Cr-Commit-Position: refs/heads/master@{#485184}
1 file changed