Prevent process form shutting down when calling WebFrameProxy::prepar…

…eForProvisionalNavigationInProcess

https://bugs.webkit.org/show_bug.cgi?id=265550
rdar://118471432

Reviewed by Pascoe.

Similar to code in WebPageProxy::triggerBrowsingContextGroupSwitchForNavigation
and WebPageProxy::continueNavigationInNewProcess, if we do anything asynchronous between
the point where we decide to use a process and the point where we actually start using it,
we need to make a shutdown preventing scope.  Otherwise, we hit the release assertion
in WebPageProxy::continueNavigationInNewProcess, which happened occasionally when loading
to and from complicated web pages with site isolation enabled.

* Source/WebKit/UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::continueNavigationInNewProcess):

Canonical link: https://commits.webkit.org/271307@main

Source/WebKit/UIProcess/WebPageProxy.cpp

@@ -4401,7 +4401,8 @@ void WebPageProxy::continueNavigationInNewProcess(API::Navigation& navigation, W
         loadParameters.shouldTreatAsContinuingLoad = navigation.currentRequestIsRedirect() ? WebCore::ShouldTreatAsContinuingLoad::YesAfterProvisionalLoadStarted : WebCore::ShouldTreatAsContinuingLoad::YesAfterNavigationPolicyDecision;
         loadParameters.frameIdentifier = frame.frameID();
 
-        frame.prepareForProvisionalNavigationInProcess(newProcess, navigation, [loadParameters = WTFMove(loadParameters), newProcess = newProcess.copyRef(), webPageID = webPageIDInProcessForDomain(RegistrableDomain(navigation.currentRequest().url()))] () mutable {
+        auto webPageID = webPageIDInProcessForDomain(RegistrableDomain(navigation.currentRequest().url()));
+        frame.prepareForProvisionalNavigationInProcess(newProcess, navigation, [loadParameters = WTFMove(loadParameters), newProcess = newProcess.copyRef(), webPageID, preventProcessShutdownScope = newProcess->shutdownPreventingScope()] () mutable {
             newProcess->send(Messages::WebPage::LoadRequest(WTFMove(loadParameters)), webPageID);
         });
         return;