Make JSONArray::at() a const member function.

third_party/WebKit/Source/platform/json/JSONValues.cpp

Index: third_party/WebKit/Source/platform/json/JSONValues.cpp
diff --git a/third_party/WebKit/Source/platform/json/JSONValues.cpp b/third_party/WebKit/Source/platform/json/JSONValues.cpp
index 9927fae52fe4e2a5bacaa9253741881ebc412396..15981e7014d7da4ad5fbc33f096e3de33a9a2b79 100644
--- a/third_party/WebKit/Source/platform/json/JSONValues.cpp
+++ b/third_party/WebKit/Source/platform/json/JSONValues.cpp
@@ -483,7 +483,7 @@ void JSONArray::pushArray(std::unique_ptr<JSONArray> value) {
   m_data.push_back(std::move(value));
 }
 
-JSONValue* JSONArray::at(size_t index) {
+JSONValue* JSONArray::at(size_t index) const {
   DCHECK_LT(index, m_data.size());
   return m_data[index].get();
 }