Fix null pointer dereference in PLSA's scrollbarStyleSource.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index 8a5fd7f3bc135dc51d8b1fe6a2e3615ff0f209be..5bc91be15a091293cbb53dda4d4fd100aa2e06c1 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1149,8 +1149,10 @@ static inline const LayoutObject& scrollbarStyleSource(
       return layoutObject;
 
     if (ShadowRoot* shadowRoot = node->containingShadowRoot()) {
-      if (shadowRoot->type() == ShadowRootType::UserAgent)
-        return *shadowRoot->host().layoutObject();
+      if (shadowRoot->type() == ShadowRootType::UserAgent) {
+        if (LayoutObject* hostLayoutObject = shadowRoot->host().layoutObject())
+          return *hostLayoutObject;
+      }
     }
   }