Defer the real work in updateCompositingLayers until it's really needed.

Source/core/rendering/RenderGeometryMap.cpp

Index: Source/core/rendering/RenderGeometryMap.cpp
diff --git a/Source/core/rendering/RenderGeometryMap.cpp b/Source/core/rendering/RenderGeometryMap.cpp
index 6f65d55b3fefe42a1f254f1e391d3f30fdc96f64..e62068c1a45d4c913a2dba65ac982b7c61cc0d19 100644
--- a/Source/core/rendering/RenderGeometryMap.cpp
+++ b/Source/core/rendering/RenderGeometryMap.cpp
@@ -118,10 +118,12 @@ FloatPoint RenderGeometryMap::mapToContainer(const FloatPoint& p, const RenderLa
     }
 
 #if !ASSERT_DISABLED
-    FloatPoint rendererMappedResult = m_mapping.last().m_renderer->localToAbsolute(p, m_mapCoordinatesFlags);
-    ASSERT(roundedIntPoint(rendererMappedResult) == roundedIntPoint(result));
-//    if (roundedIntPoint(rendererMappedResult) != roundedIntPoint(result))
-//        fprintf(stderr, "Mismatched point\n");
+    if (m_mapping.size() > 0) {
+        FloatPoint rendererMappedResult = m_mapping.last().m_renderer->localToAbsolute(p, m_mapCoordinatesFlags);
+        ASSERT(roundedIntPoint(rendererMappedResult) == roundedIntPoint(result));
+        //    if (roundedIntPoint(rendererMappedResult) != roundedIntPoint(result))
+        //        fprintf(stderr, "Mismatched point\n");
+    }
 #endif
 
     return result;
@@ -155,12 +157,14 @@ FloatQuad RenderGeometryMap::mapToContainer(const FloatRect& rect, const RenderL
     }
 
 #if !ASSERT_DISABLED
-    FloatRect rendererMappedResult = m_mapping.last().m_renderer->localToContainerQuad(rect, container, m_mapCoordinatesFlags).boundingBox();
-    // Inspector creates renderers with negative width <https://bugs.webkit.org/show_bug.cgi?id=87194>.
-    // Taking FloatQuad bounds avoids spurious assertions because of that.
-    ASSERT(enclosingIntRect(rendererMappedResult) == enclosingIntRect(FloatQuad(result).boundingBox()));
-//    if (enclosingIntRect(rendererMappedResult) != enclosingIntRect(FloatQuad(result).boundingBox()))
-//        fprintf(stderr, "Mismatched rects\n");
+    if (m_mapping.size() > 0) {
+        FloatRect rendererMappedResult = m_mapping.last().m_renderer->localToContainerQuad(rect, container, m_mapCoordinatesFlags).boundingBox();
+        // Inspector creates renderers with negative width <https://bugs.webkit.org/show_bug.cgi?id=87194>.
+        // Taking FloatQuad bounds avoids spurious assertions because of that.
+        ASSERT(enclosingIntRect(rendererMappedResult) == enclosingIntRect(FloatQuad(result).boundingBox()));
+        //    if (enclosingIntRect(rendererMappedResult) != enclosingIntRect(FloatQuad(result).boundingBox()))
+        //        fprintf(stderr, "Mismatched rects\n");
+    }
 #endif
 
     return result;