Avoid crash in |HitTestResult| when |PostLayout| is nullptr

This is probably because of <crbug.com/1152696>, where we call
|PositionForPoint| after running even handlers.

Bug: 1155733, 1150362, 1152696
Change-Id: I4ec0a685fa0507da2536d9cbff5d1e8da5076bf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574557Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834271}

third_party/blink/renderer/core/layout/hit_test_result.cc

@@ -186,8 +186,10 @@ PositionWithAffinity HitTestResult::GetPosition() const {
   // relayout?
   if (box_fragment_ &&
       RuntimeEnabledFeatures::LayoutNGFullPositionForPointEnabled() &&
-      !box_fragment_->IsLayoutObjectDestroyedOrMoved())
-    return box_fragment_->PostLayout()->PositionForPoint(LocalPoint());
+      !box_fragment_->IsLayoutObjectDestroyedOrMoved()) {
+    if (const NGPhysicalBoxFragment* fragment = box_fragment_->PostLayout())
+      return fragment->PositionForPoint(LocalPoint());
+  }
   return layout_object->PositionForPoint(LocalPoint());
 }