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}

third_party/WebKit/Source/core/editing/LayoutSelection.cpp

@@ -322,6 +322,8 @@ static SelectionPaintRange CalcSelectionPaintRange(
   DCHECK(start_layout_object);
   DCHECK(end_layout_object);
   DCHECK(start_layout_object->View() == end_layout_object->View());
+  if (!start_layout_object || !end_layout_object)
+    return SelectionPaintRange();
 
   return SelectionPaintRange(start_layout_object,
                              start_pos.ComputeEditingOffset(),