Fixing crashes in layout tests when virtual-viewport is enabled.

Source/core/inspector/InspectorLayerTreeAgent.h

Index: Source/core/inspector/InspectorLayerTreeAgent.h
diff --git a/Source/core/inspector/InspectorLayerTreeAgent.h b/Source/core/inspector/InspectorLayerTreeAgent.h
index 2d1104245a818d651ac394ee9646d48f7c6b8cd4..7fed104081b4507dffc02a3c1920f6d017e29519 100644
--- a/Source/core/inspector/InspectorLayerTreeAgent.h
+++ b/Source/core/inspector/InspectorLayerTreeAgent.h
@@ -88,6 +88,8 @@ private:
 
     explicit InspectorLayerTreeAgent(Page*);
 
+    GraphicsLayer* rootGraphicsLayer();
+
     RenderLayerCompositor* renderLayerCompositor();
     GraphicsLayer* layerById(ErrorString*, const String& layerId);
     const GraphicsContextSnapshot* snapshotById(ErrorString*, const String& snapshotId);