Use IDL argument default values in modules/indexeddb/

Source/modules/indexeddb/IDBTransaction.cpp

Index: Source/modules/indexeddb/IDBTransaction.cpp
diff --git a/Source/modules/indexeddb/IDBTransaction.cpp b/Source/modules/indexeddb/IDBTransaction.cpp
index 2f8ee47fc7c5e7ab304d13f45c7fdc3e3b4eefde..564540ce90358b69b73190855f79284609c30d7a 100644
--- a/Source/modules/indexeddb/IDBTransaction.cpp
+++ b/Source/modules/indexeddb/IDBTransaction.cpp
@@ -301,8 +301,7 @@ bool IDBTransaction::hasPendingActivity() const
 
 blink::WebIDBTransactionMode IDBTransaction::stringToMode(const String& modeString, ExceptionState& exceptionState)
 {
-    if (modeString.isNull()
-        || modeString == IDBTransaction::modeReadOnly())
+    if (modeString == IDBTransaction::modeReadOnly())
         return blink::WebIDBTransactionModeReadOnly;
     if (modeString == IDBTransaction::modeReadWrite())
         return blink::WebIDBTransactionModeReadWrite;