[Site Isolation] Fix TestWebKitAPI.ProcessSwap.DoSameSiteNavigationAf…

…terCrossSiteProvisionalLoadStarted after 278624@main

https://bugs.webkit.org/show_bug.cgi?id=274126
rdar://128023861

Reviewed by Charlie Wolfe.

I added an early return in WebPageProxy::didStartProvisionalLoadForFrameShared that I thought would only affect
behavior with site isolation on.  The assertion is hit with site isolation off, so for now just guard the behavior
change with a check to whether site isolation is on.  That fixes the regression.  When I get time to investigate
I'll see what the correct thing to do in this case with site isolation on is.

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

Canonical link: https://commits.webkit.org/278716@main

Source/WebKit/UIProcess/WebPageProxy.cpp

@@ -6004,8 +6004,9 @@ void WebPageProxy::didStartProvisionalLoadForFrameShared(Ref<WebProcessProxy>&&
     MESSAGE_CHECK_URL(process, unreachableURL);
 
     // If a provisional load has since been started in another process, ignore this message.
-    if (frame->provisionalLoadProcess().coreProcessIdentifier() != process->coreProcessIdentifier()) {
-        ASSERT(m_preferences->siteIsolationEnabled());
+    if (frame->provisionalLoadProcess().coreProcessIdentifier() != process->coreProcessIdentifier() && m_preferences->siteIsolationEnabled()) {
+        // FIXME: The API test ProcessSwap.DoSameSiteNavigationAfterCrossSiteProvisionalLoadStarted
+        // is probably not handled correctly with site isolation on.
         return;
     }