When call getAttribute, HTMLIFrame.sandbox has a bug which don't return changed value.

Source/core/html/HTMLIFrameElement.cpp

Index: Source/core/html/HTMLIFrameElement.cpp
diff --git a/Source/core/html/HTMLIFrameElement.cpp b/Source/core/html/HTMLIFrameElement.cpp
index 2e752d7493cae9b854a1a7336ba2af78f053797e..858843a7d030df51b72e12de204cf032a4cd4b52 100644
--- a/Source/core/html/HTMLIFrameElement.cpp
+++ b/Source/core/html/HTMLIFrameElement.cpp
@@ -169,6 +169,7 @@ void HTMLIFrameElement::valueChanged()
     setSandboxFlags(m_sandbox->value().isNull() ? SandboxNone : parseSandboxPolicy(m_sandbox->tokens(), invalidTokens));
     if (!invalidTokens.isNull())
         document().addConsoleMessage(ConsoleMessage::create(OtherMessageSource, ErrorMessageLevel, "Error while parsing the 'sandbox' attribute: " + invalidTokens));
+    setSynchronizedLazyAttribute(sandboxAttr, m_sandbox->value());
 }
 
 }