[Debug] Referencing m_processPool in ~WebProcessProxy() causes a …

…crash

https://bugs.webkit.org/show_bug.cgi?id=264178

Reviewed by Chris Dumez.

Cached `WebProcess`es can be destroyed as a result of clearing
`WebProcessPool`'s cache during its destruction.

In this case, referencing `m_processPool` will fail because its deletion
has already begun.

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

Canonical link: https://commits.webkit.org/270209@main

Source/WebKit/UIProcess/WebProcessProxy.cpp

@@ -339,8 +339,8 @@ WebProcessProxy::~WebProcessProxy()
     WebPasteboardProxy::singleton().removeWebProcessProxy(*this);
 
 #if HAVE(DISPLAY_LINK)
-    // Prewarmed / cached processes may not have a process pool on destruction.
-    if (RefPtr processPool = m_processPool.get())
+    // Unable to ref the process pool as it may have started destruction.
+    if (auto* processPool = m_processPool.get())
         processPool->displayLinks().stopDisplayLinks(m_displayLinkClient);
 #endif