Null-check PaintLayerScrollableArea in one more place.

Bug: 744204
Change-Id: I3a0bea65287c1b724ac0576dd7d03486789c2f96
Reviewed-on: https://chromium-review.googlesource.com/575205
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487307}

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

@@ -61,6 +61,8 @@ StickyPositionScrollingConstraints* StickyConstraintsForLayoutObject(
 
   PaintLayerScrollableArea* scrollable_area =
       ancestor_overflow_layer->GetScrollableArea();
+  if (!scrollable_area)
+    return nullptr;
   auto it = scrollable_area->GetStickyConstraintsMap().find(obj->Layer());
   if (it == scrollable_area->GetStickyConstraintsMap().end())
     return nullptr;