Reduce use of downcast<>() in DataTransfer.cpp

https://bugs.webkit.org/show_bug.cgi?id=270759

Reviewed by Anne van Kesteren.

Reduce use of downcast<>() in DataTransfer.cpp, for performance.

* Source/WebCore/dom/DataTransfer.cpp:
(WebCore::DataTransfer::setDataFromItemList):
(WebCore::DataTransfer::commitToPasteboard):

Canonical link: https://commits.webkit.org/275894@main

Source/WebCore/dom/DataTransfer.cpp

@@ -261,10 +261,10 @@ void DataTransfer::setData(Document& document, const String& type, const String&
 void DataTransfer::setDataFromItemList(Document& document, const String& type, const String& data)
 {
     ASSERT(canWriteData());
-    RELEASE_ASSERT(is<StaticPasteboard>(*m_pasteboard));
 
+    auto& pasteboard = checkedDowncast<StaticPasteboard>(*m_pasteboard);
     if (!DeprecatedGlobalSettings::customPasteboardDataEnabled()) {
-        m_pasteboard->writeString(type, data);
+        pasteboard.writeString(type, data);
         return;
     }
 
@@ -284,10 +284,10 @@ void DataTransfer::setDataFromItemList(Document& document, const String& type, c
     }
 
     if (sanitizedData != data)
-        downcast<StaticPasteboard>(*m_pasteboard).writeStringInCustomData(type, data);
+        pasteboard.writeStringInCustomData(type, data);
 
     if (Pasteboard::isSafeTypeForDOMToReadAndWrite(type) && !sanitizedData.isNull())
-        m_pasteboard->writeString(type, sanitizedData);
+        pasteboard.writeString(type, sanitizedData);
 }
 
 void DataTransfer::updateFileList(ScriptExecutionContext* context)
@@ -451,7 +451,7 @@ Ref<DataTransfer> DataTransfer::createForInputEvent(const String& plainText, con
 
 void DataTransfer::commitToPasteboard(Pasteboard& nativePasteboard)
 {
-    ASSERT(is<StaticPasteboard>(*m_pasteboard) && !is<StaticPasteboard>(nativePasteboard));
+    ASSERT(!is<StaticPasteboard>(nativePasteboard));
     auto& staticPasteboard = downcast<StaticPasteboard>(*m_pasteboard);
     if (!staticPasteboard.hasNonDefaultData()) {
         // We clear the platform pasteboard here to ensure that the pasteboard doesn't contain any data