[ios] Fix transition type for same-document back/forward navigation.

The ui::PAGE_TRANSITION_FORWARD_BACK bit has always been set for
different-document back/forward navigation. It's not clear why
same-document back/forward navigation should be treated differently.
Making this change matches the Blink behavior (so should be low risk)
and removes a special case.

Bug: 798832
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I39e7a28caa9069cbb12ebdc965918792b36b77b2
Reviewed-on: https://chromium-review.googlesource.com/847940
Commit-Queue: Danyao Wang <danyao@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527105}

ios/web/navigation/legacy_navigation_manager_impl.mm

@@ -305,6 +305,9 @@ void LegacyNavigationManagerImpl::FinishGoToIndex(
   NavigationItem* to_item = items[index].get();
   NavigationItem* previous_item = [session_controller_ currentItem];
 
+  to_item->SetTransitionType(ui::PageTransitionFromInt(
+      to_item->GetTransitionType() | ui::PAGE_TRANSITION_FORWARD_BACK));
+
   bool same_document_navigation =
       [session_controller_ isSameDocumentNavigationBetweenItem:previous_item
                                                        andItem:to_item];
@@ -314,11 +317,6 @@ void LegacyNavigationManagerImpl::FinishGoToIndex(
   } else {
     [session_controller_ discardNonCommittedItems];
     [session_controller_ setPendingItemIndex:index];
-
-    NavigationItemImpl* pending_item = [session_controller_ pendingItem];
-    pending_item->SetTransitionType(ui::PageTransitionFromInt(
-        pending_item->GetTransitionType() | ui::PAGE_TRANSITION_FORWARD_BACK));
-
     delegate_->LoadCurrentItem();
   }
 }

ios/web/navigation/navigation_manager_impl_unittest.mm

@@ -2293,12 +2293,8 @@ TEST_P(NavigationManagerTest, GoToIndexSameDocument) {
   }
 
   navigation_manager()->GoToIndex(0);
-  // Preserve the existing behavior of legacy navigation manager for change
-  // management, even though it seems like a bug that the back-forward
-  // transition bit is not set for same-document history navigation.
-  EXPECT_EQ(GetParam() == TEST_WK_BASED_NAVIGATION_MANAGER,
-            (navigation_manager()->GetItemAtIndex(0)->GetTransitionType() &
-             ui::PAGE_TRANSITION_FORWARD_BACK) > 0);
+  EXPECT_TRUE(navigation_manager()->GetItemAtIndex(0)->GetTransitionType() &
+              ui::PAGE_TRANSITION_FORWARD_BACK);
 
   if (GetParam() == TEST_LEGACY_NAVIGATION_MANAGER) {
     EXPECT_EQ(0, navigation_manager()->GetLastCommittedItemIndex());