Disallow back forward navigations during async session restoration.

Slim Navigation Manager asynchronously restores the session. During
session restoration it's not possible to perform back-forward
navigation. This change prevents all delta navigations until the session
is restored.

Bug: 877671
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I801a99182582118862d03796532588f1495de9c6
Reviewed-on: https://chromium-review.googlesource.com/1187677Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586041}

ios/web/navigation/wk_based_navigation_manager_impl.mm

@@ -381,6 +381,9 @@ bool WKBasedNavigationManagerImpl::CanGoForward() const {
 }
 
 bool WKBasedNavigationManagerImpl::CanGoToOffset(int offset) const {
+  if (is_restore_session_in_progress_)
+    return false;
+
   // If the last committed item is the empty window.open item, no back-forward
   // navigation is allowed.
   if (empty_window_open_item_) {

ios/web/web_state/web_state_unittest.mm

@@ -284,7 +284,12 @@ TEST_F(WebStateTest, RestoreLargeSession) {
 
   // Verify that session was fully restored.
   EXPECT_TRUE(WaitUntilConditionOrTimeout(kWaitForPageLoadTimeout, ^{
-    return navigation_manager->GetItemCount() == kItemCount;
+    bool restored = navigation_manager->GetItemCount() == kItemCount;
+    if (!restored) {
+      EXPECT_FALSE(navigation_manager->CanGoBack());
+      EXPECT_FALSE(navigation_manager->CanGoForward());
+    }
+    return restored;
   }));
   EXPECT_EQ(kItemCount, navigation_manager->GetItemCount());
 }