Don't call RenderObject::enclosingLayer on a null pointer

Source/core/rendering/RenderObject.cpp

Index: Source/core/rendering/RenderObject.cpp
diff --git a/Source/core/rendering/RenderObject.cpp b/Source/core/rendering/RenderObject.cpp
index 31a847fe366c0a530aead2289ed17e7cf67fc4d6..d41640796a782dc3a601a73d5a4b1f7def042229 100644
--- a/Source/core/rendering/RenderObject.cpp
+++ b/Source/core/rendering/RenderObject.cpp
@@ -529,10 +529,8 @@ RenderLayer* RenderObject::enclosingLayer() const
         if (current->hasLayer())
             return toRenderLayerModelObject(current)->layer();
     }
-    // This line of code should be unreachable. However, if |this| is zero, we
-    // can reach this line. Of course, it's undefined behavior to call a member
-    // function on a zero pointer, but that doesn't stop
-    // FrameView::paintContents. :)
+    // This function can return zero only for RenderObjects that have not yet been added to the tree.
+    ASSERT(!m_parent && !isRenderView());

[esprehn, 2014/03/27 23:54:28]

I'd ASSERT(isRooted()) at the top and ASSERT_NOT_REACHED() here and make
everyone stop calling this on things outside the tree.

     return 0;
 }