Avoid WTF::Vector::at() and operator[] in core/events.

third_party/WebKit/Source/core/events/TreeScopeEventContext.cpp

Index: third_party/WebKit/Source/core/events/TreeScopeEventContext.cpp
diff --git a/third_party/WebKit/Source/core/events/TreeScopeEventContext.cpp b/third_party/WebKit/Source/core/events/TreeScopeEventContext.cpp
index 93bfbea6fc27725f3bf5bb08f683ae86a154b5ec..b7ab3e248e4c25e5831fc946452b6048a317381f 100644
--- a/third_party/WebKit/Source/core/events/TreeScopeEventContext.cpp
+++ b/third_party/WebKit/Source/core/events/TreeScopeEventContext.cpp
@@ -112,10 +112,10 @@ int TreeScopeEventContext::calculateTreeOrderAndSetNearestAncestorClosedTree(
       (rootNode().isShadowRoot() && !toShadowRoot(rootNode()).isOpenOrV0())
           ? this
           : nearestAncestorClosedTreeScopeEventContext;
-  for (size_t i = 0; i < m_children.size(); ++i)
-    orderNumber =
-        m_children[i]->calculateTreeOrderAndSetNearestAncestorClosedTree(
-            orderNumber + 1, containingClosedShadowTree());
+  for (const auto& context : m_children) {
+    orderNumber = context->calculateTreeOrderAndSetNearestAncestorClosedTree(
+        orderNumber + 1, containingClosedShadowTree());
+  }
   m_postOrder = orderNumber + 1;
 
   return orderNumber + 1;