Null-check scrollable area when invalidating sticky constraints.

BUG=678452

Review-Url: https://codereview.chromium.org/2617433005
Cr-Commit-Position: refs/heads/master@{#441983}

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

@@ -401,9 +401,11 @@ void LayoutBoxModelObject::styleDidChange(StyleDifference diff,
         if (layer()) {
           DisableCompositingQueryAsserts disabler;
           if (const PaintLayer* ancestorOverflowLayer =
-                  layer()->ancestorOverflowLayer())
-            ancestorOverflowLayer->getScrollableArea()
-                ->invalidateStickyConstraintsFor(layer());
+                  layer()->ancestorOverflowLayer()) {
+            if (PaintLayerScrollableArea* scrollableArea =
+                    ancestorOverflowLayer->getScrollableArea())
+              scrollableArea->invalidateStickyConstraintsFor(layer());
+          }
         }
 
         // TODO(pdr): When slimming paint v2 is enabled, we will need to