REGRESSION(269895@main):[WPE] 52 new API tests failures

https://bugs.webkit.org/show_bug.cgi?id=263974

Unreviewed, very partial revert of 269895@main to address the crashes on WPE.
It is unclear why WPE crashes when protecting the process pool in this case,
since we don't see the WebProcessPool destructor in the stack trace.

* Source/WebKit/UIProcess/WebProcessProxy.cpp:
(WebKit::WebProcessProxy::~WebProcessProxy):

Canonical link: https://commits.webkit.org/270054@main

Source/WebKit/UIProcess/WebProcessProxy.cpp

@@ -337,7 +337,7 @@ WebProcessProxy::~WebProcessProxy()
     WebPasteboardProxy::singleton().removeWebProcessProxy(*this);
 
 #if HAVE(DISPLAY_LINK)
-    protectedProcessPool()->displayLinks().stopDisplayLinks(m_displayLinkClient);
+    processPool().displayLinks().stopDisplayLinks(m_displayLinkClient);
 #endif
 
     auto isResponsiveCallbacks = WTFMove(m_isResponsiveCallbacks);