[PE] Fix crash in ScrollableAreaPainter::PaintOverflowControls()

Just a speculative fix.
Still don't know why a scrollable box doesn't have paint properties.

Bug: 849278
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I4add45ef22ae71d6cc0ec617b4405fb5fdf2145f
Reviewed-on: https://chromium-review.googlesource.com/1090037Reviewed-by: Tien-Ren Chen <trchen@chromium.org>
Commit-Queue: Tien-Ren Chen <trchen@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565422}

third_party/blink/renderer/core/paint/scrollable_area_painter.cc

@@ -180,13 +180,17 @@ void ScrollableAreaPainter::PaintOverflowControls(
     const auto& box = *GetScrollableArea().GetLayoutBox();
     if (const auto* fragment = paint_info.FragmentToPaint(box)) {
       const auto* properties = fragment->PaintProperties();
+      // TODO(crbug.com/849278): Remove either the DCHECK or the if condition
+      // when we figure out in what cases that the box doesn't have properties.
       DCHECK(properties);
+      if (properties) {
         if (const auto* clip = properties->OverflowControlsClip()) {
           scoped_paint_chunk_properties.emplace(
               context.GetPaintController(), clip, box,
               DisplayItem::kClipLayerOverflowControls);
         }
       }
+    }
   } else {
     IntRect clip_rect(adjusted_paint_offset,
                       GetScrollableArea().Layer()->PixelSnappedSize());