Fix OOPIF-based <webview>.

content/common/site_isolation_policy.cc

Index: content/common/site_isolation_policy.cc
diff --git a/content/common/site_isolation_policy.cc b/content/common/site_isolation_policy.cc
index 60288700ca7141239288fe2fedb8a4af593b75d9..60deee961811ac2a2d98720fd899df37573be46f 100644
--- a/content/common/site_isolation_policy.cc
+++ b/content/common/site_isolation_policy.cc
@@ -6,6 +6,7 @@
 
 #include "base/command_line.h"
 #include "base/lazy_instance.h"
+#include "content/public/common/browser_plugin_guest_mode.h"
 #include "content/public/common/browser_side_navigation_policy.h"
 #include "content/public/common/content_client.h"
 #include "content/public/common/content_switches.h"
@@ -16,7 +17,8 @@ namespace content {
 bool SiteIsolationPolicy::AreCrossProcessFramesPossible() {
   return base::CommandLine::ForCurrentProcess()->HasSwitch(
              switches::kSitePerProcess) ||
-         GetContentClient()->IsSupplementarySiteIsolationModeEnabled();
+         GetContentClient()->IsSupplementarySiteIsolationModeEnabled() ||
+         BrowserPluginGuestMode::UseCrossProcessFramesForGuests();
 }
 
 // static