Fetch API: Fix behavior when Request constructor is passed an undefined referrer

third_party/WebKit/Source/modules/fetch/RequestInit.cpp

Index: third_party/WebKit/Source/modules/fetch/RequestInit.cpp
diff --git a/third_party/WebKit/Source/modules/fetch/RequestInit.cpp b/third_party/WebKit/Source/modules/fetch/RequestInit.cpp
index 54a76a215bdb1497bd74102830eef4e701a0dc90..7ece93648761c7bc426ca423b31f32f923c3bd11 100644
--- a/third_party/WebKit/Source/modules/fetch/RequestInit.cpp
+++ b/third_party/WebKit/Source/modules/fetch/RequestInit.cpp
@@ -45,8 +45,8 @@ RequestInit::RequestInit(ExecutionContext* context,
   areAnyMembersSet |= DictionaryHelper::get(options, "mode", mode);
   areAnyMembersSet |= DictionaryHelper::get(options, "redirect", redirect);
   AtomicString referrerString;
-  bool isReferrerStringSet =
-      DictionaryHelper::get(options, "referrer", referrerString);
+  bool isReferrerStringSet = DictionaryHelper::getWithUndefinedCheck(
+      options, "referrer", referrerString);
   areAnyMembersSet |= isReferrerStringSet;
   areAnyMembersSet |= DictionaryHelper::get(options, "integrity", integrity);
   AtomicString referrerPolicyString;