Fix another source of flakiness in ErrorPageTest

chrome/test/automation/tab_proxy.h

Index: chrome/test/automation/tab_proxy.h
diff --git a/chrome/test/automation/tab_proxy.h b/chrome/test/automation/tab_proxy.h
index 9b7232f853daa51df7ca444ed9f16730ccb2568c..c8bb83e954f185e82abd8fefe12b9a78800afe98 100644
--- a/chrome/test/automation/tab_proxy.h
+++ b/chrome/test/automation/tab_proxy.h
@@ -120,10 +120,20 @@ class TabProxy : public AutomationResourceProxy {
   // hence blocks until the navigation completes.
   AutomationMsg_NavigationResponseValues GoBack();
 
+  // Equivalent to hitting the Back button. This is a synchronous call and
+  // hence blocks until the |number_of_navigations| navigations complete.
+  AutomationMsg_NavigationResponseValues GoBackBlockUntilNavigationsComplete(
+      int number_of_navigations);
+
   // Equivalent to hitting the Forward button. This is a synchronous call and
   // hence blocks until the navigation completes.
   AutomationMsg_NavigationResponseValues GoForward();
 
+  // Equivalent to hitting the Forward button. This is a synchronous call and
+  // hence blocks until the |number_of_navigations| navigations complete.
+  AutomationMsg_NavigationResponseValues GoForwardBlockUntilNavigationsComplete(
+      int number_of_navigations);
+
   // Equivalent to hitting the Reload button. This is a synchronous call and
   // hence blocks until the navigation completes.
   AutomationMsg_NavigationResponseValues Reload();