Add a const version of JSONObject::cast()

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

Index: third_party/WebKit/Source/platform/json/JSONValues.h
diff --git a/third_party/WebKit/Source/platform/json/JSONValues.h b/third_party/WebKit/Source/platform/json/JSONValues.h
index 4a37cd7913cff7254aa28ade9b781bf7856ca8c2..cb666a840d46d64fe7250a1f8590aa7cab364c23 100644
--- a/third_party/WebKit/Source/platform/json/JSONValues.h
+++ b/third_party/WebKit/Source/platform/json/JSONValues.h
@@ -177,6 +177,12 @@ class PLATFORM_EXPORT JSONObject : public JSONValue {
     return static_cast<JSONObject*>(value);
   }
 
+  static const JSONObject* cast(const JSONValue* value) {
+    if (!value || value->getType() != TypeObject)
+      return nullptr;
+    return static_cast<const JSONObject*>(value);
+  }
+
   static std::unique_ptr<JSONObject> from(std::unique_ptr<JSONValue> value) {
     auto maybeObject = WTF::wrapUnique(JSONObject::cast(value.get()));
     if (maybeObject)