Make ensureCompositedMappingLayer() to be a void function.

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index ed86a0c6dd4c85956900d5dc01c66957e3cef676..c379432c1eaf7ee53a7a65ed19653c06ee05317d 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -283,8 +283,6 @@ public:
 
     // Don't null check this.
     CompositedLayerMapping* compositedLayerMapping() const;
-    // FIXME: This should return a reference.
-    CompositedLayerMapping* ensureCompositedLayerMapping();
     GraphicsLayer* graphicsLayerBacking() const;
     GraphicsLayer* graphicsLayerBackingForScrolling() const;
     // NOTE: If you are using hasCompositedLayerMapping to determine the state of compositing for this layer,
@@ -292,6 +290,7 @@ public:
     // then you may have incorrect logic. Use compositingState() instead.
     // FIXME: This is identical to null checking compositedLayerMapping(), why not just call that?
     bool hasCompositedLayerMapping() const { return m_compositedLayerMapping.get(); }
+    void ensureCompositedLayerMapping();
     void clearCompositedLayerMapping(bool layerBeingDestroyed = false);
     CompositedLayerMapping* groupedMapping() const { return m_groupedMapping; }
     void setGroupedMapping(CompositedLayerMapping* groupedMapping, bool layerBeingDestroyed = false);