Disable cross-process navigations in --single-process.

content/browser/site_instance_impl.cc

Index: content/browser/site_instance_impl.cc
diff --git a/content/browser/site_instance_impl.cc b/content/browser/site_instance_impl.cc
index 7a5ed8e4c3f0d9e64296a6553d8d9aecc5bd345e..08cb4e3cf76d89fda4ca112a2bcf327633502c6e 100644
--- a/content/browser/site_instance_impl.cc
+++ b/content/browser/site_instance_impl.cc
@@ -4,6 +4,7 @@
 
 #include "content/browser/site_instance_impl.h"
 
+#include "base/command_line.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "content/browser/browsing_instance.h"
@@ -16,6 +17,7 @@
 #include "content/public/browser/content_browser_client.h"
 #include "content/public/browser/render_process_host_factory.h"
 #include "content/public/browser/web_ui_controller_factory.h"
+#include "content/public/common/content_switches.h"
 #include "content/public/common/url_constants.h"
 #include "net/base/registry_controlled_domains/registry_controlled_domain.h"
 
@@ -434,6 +436,12 @@ bool SiteInstanceImpl::DoesSiteRequireDedicatedProcess(
 // static
 bool SiteInstanceImpl::ShouldLockToOrigin(BrowserContext* browser_context,
                                           GURL site_url) {
+  // Don't lock to origin in --single-process mode, since this mode puts
+  // cross-site pages into the same process.
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kSingleProcess))
+    return false;
+
   if (!DoesSiteRequireDedicatedProcess(browser_context, site_url))
     return false;