[UnifedPDF] Occasional crash at GraphicsLayerCA::setNeedsDisplayInRec…

…t() with slow-rendering PDFs

https://bugs.webkit.org/show_bug.cgi?id=273661
rdar://127443945

Reviewed by Abrar Rahman Protyasha.

We should use `unparentAndClear()` on the GraphicsLayers to both unparent them, and clear their
GraphicsLayerClient. Do this on the layers which draw content, since those are the ones whose
clients can get called after a delay.

* Source/WebKit/WebProcess/Plugins/PDF/UnifiedPDF/UnifiedPDFPlugin.mm:
(WebKit::UnifiedPDFPlugin::teardown):

Canonical link: https://commits.webkit.org/278298@main

Source/WebKit/WebProcess/Plugins/PDF/UnifiedPDF/UnifiedPDFPlugin.mm

@@ -182,8 +182,11 @@ static String mutationObserverNotificationString()
 
     PDFPluginBase::teardown();
 
-    if (m_rootLayer)
-        m_rootLayer->removeFromParent();
+    GraphicsLayer::unparentAndClear(m_rootLayer);
+    GraphicsLayer::unparentAndClear(m_contentsLayer);
+#if ENABLE(UNIFIED_PDF_SELECTION_LAYER)
+    GraphicsLayer::unparentAndClear(m_selectionLayer);
+#endif
 
     RefPtr page = this->page();
     if (m_scrollingNodeID && page) {