Measure usage of FormData.append(name, blob, filename)

Source/core/html/FormData.idl

Index: Source/core/html/FormData.idl
diff --git a/Source/core/html/FormData.idl b/Source/core/html/FormData.idl
index 01efe14eeb0c9630a23a7742f1b71cc23ad2d7e1..dccbd43de03f2eca2feeff50b9b4c2493fe2bac0 100644
--- a/Source/core/html/FormData.idl
+++ b/Source/core/html/FormData.idl
@@ -40,8 +40,8 @@ typedef (File or USVString) FormDataEntryValue;
     ImplementedAs=DOMFormData,
 ] interface FormData {
     // TODO(philipj): The value argument should be FormDataEntryValue and there
-    // should be no optional filename argument.
-    void append(USVString name, Blob value, optional USVString filename);
+    // should be no optional filename argument. crbug.com/498790
+    [CallWith=ExecutionContext] void append(USVString name, Blob value, optional USVString filename);
     void append(USVString name, USVString value);
 
     [RuntimeEnabled=FormDataNewMethods, ImplementedAs=deleteEntry] void delete(USVString name);