Fix SelectionController for NG multicol

Following up r824533 <crrev.com/c/2515399> which traverses
fragments for |PositionForPoint|, this patch fixes
|SelectionController| to traverse fragments without going
through |LayoutObject|.

It looks like the code in |SelectionController| has slight
difference from |HitTestResult::GetPosition()| but it does not
look intentional.

The fix is needed to support block fragmentation in
<crrev.com/c/2514048>.

Bug: 829028
Change-Id: I35060e5e2030e3282c60cb11b590105c11f69ca3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522799Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824887}

third_party/blink/renderer/core/editing/selection_controller.cc

@@ -125,9 +125,7 @@ bool CanMouseDownStartSelect(Node* node) {
 PositionInFlatTreeWithAffinity PositionWithAffinityOfHitTestResult(
     const HitTestResult& hit_test_result) {
   return FromPositionInDOMTree<EditingInFlatTreeStrategy>(
-      hit_test_result.InnerPossiblyPseudoNode()
-          ->GetLayoutObject()
-          ->PositionForPoint(hit_test_result.LocalPoint()));
+      hit_test_result.GetPosition());
 }
 
 DocumentMarker* SpellCheckMarkerAtPosition(