Add more smart pointers to RemoteDisplayListRecorder.cpp

https://bugs.webkit.org/show_bug.cgi?id=260672
rdar://114401181

Reviewed by Ryosuke Niwa.

* Source/WebKit/GPUProcess/graphics/RemoteDisplayListRecorder.cpp:
(WebKit::RemoteDisplayListRecorder::setState):

Canonical link: https://commits.webkit.org/267255@main

Source/WebKit/GPUProcess/graphics/RemoteDisplayListRecorder.cpp

@@ -144,7 +144,7 @@ void RemoteDisplayListRecorder::setState(DisplayList::SetState&& item)
         auto gradientIdentifier = brush.gradientIdentifier();
         if (!gradientIdentifier)
             return true;
-        auto gradient = resourceCache().cachedGradient(*gradientIdentifier);
+        RefPtr gradient = resourceCache().cachedGradient(*gradientIdentifier);
         if (!gradient) {
             ASSERT_NOT_REACHED();
             return false;