Make 270849@main have no effect when site isolation is off

https://bugs.webkit.org/show_bug.cgi?id=265063
rdar://118574473

Unreviewed.

It was supposed to only change behavior when site isolation was on.
It broke the tests http/tests/inspector/target/provisional-load-cancels-previous-load.html
and TestWebKitAPI.ProcessSwap.QuickBackForwardNavigationWithPSON.
This fixes those two tests and restores existing behavior with site isolation off.
There is still work to do when site isolation is on.  A lot of work.

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

Canonical link: https://commits.webkit.org/270912@main

Source/WebKit/UIProcess/WebPageProxy.cpp

@@ -6647,7 +6647,12 @@ void WebPageProxy::decidePolicyForNavigationAction(Ref<WebProcessProxy>&& proces
     else if (m_needsInitialLinkDecorationFilteringData)
         sendCachedLinkDecorationFilteringData();
 #endif
-    Ref frameProcessBeforeNavigation { frame.provisionalFrame() ? frame.provisionalFrame()->process() : frame.isMainFrame() && m_provisionalPage ? m_provisionalPage->process() : frame.process() };
+
+    // FIXME: preferences().siteIsolationEnabled() here indicates there's an issue.
+    // See rdar://118574473 and rdar://118575766 for the kinds of things that might
+    // be broken with site isolation enabled.
+    Ref frameProcessBeforeNavigation { frame.provisionalFrame() ? frame.provisionalFrame()->process() : preferences().siteIsolationEnabled()  && frame.isMainFrame() && m_provisionalPage ? m_provisionalPage->process() : frame.process() };
+
     Ref listener = frame.setUpPolicyListenerProxy([this, protectedThis = Ref { *this }, frameProcessBeforeNavigation = WTFMove(frameProcessBeforeNavigation), processInitiatingNavigation = process, frame = Ref { frame }, sender = WTFMove(sender), navigation, navigationAction, frameInfo] (PolicyAction policyAction, API::WebsitePolicies* policies, ProcessSwapRequestedByClient processSwapRequestedByClient, RefPtr<SafeBrowsingWarning>&& safeBrowsingWarning, std::optional<NavigatingToAppBoundDomain> isAppBoundDomain, WasNavigationIntercepted wasNavigationIntercepted) mutable {
         WEBPAGEPROXY_RELEASE_LOG(Loading, "decidePolicyForNavigationAction: listener called: frameID=%llu, isMainFrame=%d, navigationID=%llu, policyAction=%u, safeBrowsingWarning=%d, isAppBoundDomain=%d, wasNavigationIntercepted=%d", frame->frameID().object().toUInt64(), frame->isMainFrame(), navigation ? navigation->navigationID() : 0, (unsigned)policyAction, !!safeBrowsingWarning, !!isAppBoundDomain, wasNavigationIntercepted == WasNavigationIntercepted::Yes);