REGRESSION (255929@main): Some Facebook links cannot be opened in Safari

https://bugs.webkit.org/show_bug.cgi?id=247351
rdar://101818726

Reviewed by Tim Horton and John Wilander.

Guard the new call to `WebFrameProxy::swapToProcess()` (which is currently a no-op) behind the
experimental `SiteIsolationEnabled` feature flag.

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

Canonical link: https://commits.webkit.org/256213@main

Source/WebKit/UIProcess/WebPageProxy.cpp

@@ -3763,13 +3763,13 @@ void WebPageProxy::continueNavigationInNewProcess(API::Navigation& navigation, W
     RefPtr websitePolicies = navigation.websitePolicies();
     bool isServerSideRedirect = shouldTreatAsContinuingLoad == ShouldTreatAsContinuingLoad::YesAfterNavigationPolicyDecision && navigation.currentRequestIsRedirect();
     bool isProcessSwappingOnNavigationResponse = shouldTreatAsContinuingLoad == ShouldTreatAsContinuingLoad::YesAfterProvisionalLoadStarted;
-    if (frame.isMainFrame())
-        m_provisionalPage = makeUnique<ProvisionalPageProxy>(*this, WTFMove(newProcess), WTFMove(suspendedPage), navigation.navigationID(), isServerSideRedirect, navigation.currentRequest(), processSwapRequestedByClient, isProcessSwappingOnNavigationResponse, websitePolicies.get());
-    else {
+    if (!frame.isMainFrame() && preferences().siteIsolationEnabled()) {
         frame.swapToProcess(WTFMove(newProcess));
         return;
     }
 
+    m_provisionalPage = makeUnique<ProvisionalPageProxy>(*this, WTFMove(newProcess), WTFMove(suspendedPage), navigation.navigationID(), isServerSideRedirect, navigation.currentRequest(), processSwapRequestedByClient, isProcessSwappingOnNavigationResponse, websitePolicies.get());
+
     auto continuation = [this, protectedThis = Ref { *this }, navigation = Ref { navigation }, shouldTreatAsContinuingLoad, websitePolicies = WTFMove(websitePolicies), existingNetworkResourceLoadIdentifierToResume]() mutable {
         if (auto* item = navigation->targetItem()) {
             LOG(Loading, "WebPageProxy %p continueNavigationInNewProcess to back item URL %s", this, item->url().utf8().data());