Fix another source of flakiness in ErrorPageTest

chrome/test/automation/tab_proxy.cc

Index: chrome/test/automation/tab_proxy.cc
diff --git a/chrome/test/automation/tab_proxy.cc b/chrome/test/automation/tab_proxy.cc
index f59f821baa8b79f444028074b10743d37aa34d3d..6a1cb0a4d3cfa565cdbace2e793e9209b4a8c278 100644
--- a/chrome/test/automation/tab_proxy.cc
+++ b/chrome/test/automation/tab_proxy.cc
@@ -160,22 +160,35 @@ bool TabProxy::NeedsAuth() const {
 }
 
 AutomationMsg_NavigationResponseValues TabProxy::GoBack() {
+  return GoBackBlockUntilNavigationsComplete(1);
+}
+
+AutomationMsg_NavigationResponseValues
+    TabProxy::GoBackBlockUntilNavigationsComplete(int number_of_navigations) {
   if (!is_valid())
     return AUTOMATION_MSG_NAVIGATION_ERROR;
 
   AutomationMsg_NavigationResponseValues navigate_response =
       AUTOMATION_MSG_NAVIGATION_ERROR;
-  sender_->Send(new AutomationMsg_GoBack(0, handle_, &navigate_response));
+  sender_->Send(new AutomationMsg_GoBackBlockUntilNavigationsComplete(
+      0, handle_, number_of_navigations, &navigate_response));
   return navigate_response;
 }
 
 AutomationMsg_NavigationResponseValues TabProxy::GoForward() {
+  return GoForwardBlockUntilNavigationsComplete(1);
+}
+
+AutomationMsg_NavigationResponseValues
+    TabProxy::GoForwardBlockUntilNavigationsComplete(
+        int number_of_navigations) {
   if (!is_valid())
     return AUTOMATION_MSG_NAVIGATION_ERROR;
 
   AutomationMsg_NavigationResponseValues navigate_response =
       AUTOMATION_MSG_NAVIGATION_ERROR;
-  sender_->Send(new AutomationMsg_GoForward(0, handle_, &navigate_response));
+  sender_->Send(new AutomationMsg_GoForwardBlockUntilNavigationsComplete(
+      0, handle_, number_of_navigations, &navigate_response));
   return navigate_response;
 }