[ios] Allow having multiple session windows in a session.

ios/chrome/browser/test/perf_test_with_bvc_ios.mm

Index: ios/chrome/browser/test/perf_test_with_bvc_ios.mm
diff --git a/ios/chrome/browser/test/perf_test_with_bvc_ios.mm b/ios/chrome/browser/test/perf_test_with_bvc_ios.mm
index 146c521a2561eaea18f8de067fc7442b96feee95..d8a056cdd2c866a7cb9b2276e149f20395af740c 100644
--- a/ios/chrome/browser/test/perf_test_with_bvc_ios.mm
+++ b/ios/chrome/browser/test/perf_test_with_bvc_ios.mm
@@ -14,6 +14,7 @@
 #include "ios/chrome/browser/bookmarks/bookmark_model_factory.h"
 #include "ios/chrome/browser/browser_state/test_chrome_browser_state_manager.h"
 #include "ios/chrome/browser/search_engines/template_url_service_factory.h"
+#import "ios/chrome/browser/sessions/session_ios.h"
 #import "ios/chrome/browser/sessions/session_service_ios.h"
 #import "ios/chrome/browser/sessions/session_window_ios.h"
 #import "ios/chrome/browser/tabs/tab_model.h"
@@ -118,17 +119,18 @@ void PerfTestWithBVC::SetUp() {
   // Use the session to create a window which will contain the tab models.
   NSString* state_path = base::SysUTF8ToNSString(
       chrome_browser_state_->GetStatePath().AsUTF8Unsafe());
-  SessionWindowIOS* session_window = [[SessionServiceIOS sharedService]
-      loadSessionWindowFromDirectory:state_path];
+  SessionIOS* session =
+      [[SessionServiceIOS sharedService] loadSessionFromDirectory:state_path];
+  DCHECK_EQ(session.sessionWindows.count, 1u);
 
   // Tab models. The off-the-record (OTR) tab model is required for the stack
   // view controller, which is created in OpenStackView().
   tab_model_.reset([[TabModel alloc]
-      initWithSessionWindow:session_window
+      initWithSessionWindow:session.sessionWindows[0]
              sessionService:[SessionServiceIOS sharedService]
                browserState:chrome_browser_state_.get()]);
   otr_tab_model_.reset([[TabModel alloc]
-      initWithSessionWindow:session_window
+      initWithSessionWindow:session.sessionWindows[0]
              sessionService:[SessionServiceIOS sharedService]
                browserState:chrome_browser_state_
                                 ->GetOffTheRecordChromeBrowserState()]);