Need to keep the document alive in KeyframeEffect::processKeyframes()

https://bugs.webkit.org/show_bug.cgi?id=240677
<rdar://93513759>

Reviewed by Saam Barati.

Since the Document is used throughout KeyframeEffect::processKeyframes and it's provided
as a simple reference, we must ensure we keep it alive with a Ref since JS code in a custom
iterator for the keyframes object could cause it to be torn down.

* Source/WebCore/animation/KeyframeEffect.cpp:
(WebCore::KeyframeEffect::processKeyframes):

Canonical link: https://commits.webkit.org/250756@main
git-svn-id: https://svn.webkit.org/repository/webkit/trunk@294497 268f45cc-cd09-0410-ab3c-d52691b4dbfc

Source/WebCore/animation/KeyframeEffect.cpp

@@ -726,6 +726,8 @@ void KeyframeEffect::keyframesRuleDidChange()
 
 ExceptionOr<void> KeyframeEffect::processKeyframes(JSGlobalObject& lexicalGlobalObject, Document& document, Strong<JSObject>&& keyframesInput)
 {
+    Ref protectedDocument { document };
+
     // 1. If object is null, return an empty sequence of keyframes.
     if (!keyframesInput.get())
         return { };