[ios] Fixed GetIndexForOffset for pending transient items.

GetIndexForOffset() should decrease resulting index by 1 for pending
transient items to allow going back from interstitials that were added
to the middle of the stack.

BUG=677190

Review-Url: https://codereview.chromium.org/2600263002
Cr-Commit-Position: refs/heads/master@{#440808}

ios/web/navigation/navigation_manager_impl.mm

@@ -344,9 +344,10 @@ int NavigationManagerImpl::GetIndexForOffset(int offset) const {
                    : static_cast<int>([session_controller_ pendingEntryIndex]);
 
   if (offset < 0) {
-    if (GetTransientItem()) {
-      // Going back from transient item is a matter of discarding it and there
-      // is no need to move navigation index back.
+    if (GetTransientItem() && [session_controller_ pendingEntryIndex] == -1) {
+      // Going back from transient item that added to the end navigation stack
+      // is a matter of discarding it as there is no need to move navigation
+      // index back.
       offset++;
     }
 

ios/web/navigation/navigation_manager_impl_unittest.mm

@@ -459,11 +459,13 @@ TEST_F(NavigationManagerTest, OffsetsWithoutPendingIndex) {
   EXPECT_EQ(-999999998, navigation_manager()->GetIndexForOffset(-1000000000));
   EXPECT_EQ(1000000004, navigation_manager()->GetIndexForOffset(1000000000));
 
-  // Test with existing transient entry.
+  // Test with existing transient entry in the end of the stack.
+  [session_controller() goToEntryAtIndex:4];
+  [session_controller() setPendingEntryIndex:-1];
   [session_controller() addTransientEntryWithURL:GURL("http://www.url.com")];
   ASSERT_EQ(5, navigation_manager()->GetItemCount());
-  ASSERT_EQ(1, navigation_manager()->GetCurrentItemIndex());
-  ASSERT_EQ(4, navigation_manager()->GetPendingItemIndex());
+  ASSERT_EQ(4, navigation_manager()->GetCurrentItemIndex());
+  ASSERT_EQ(-1, navigation_manager()->GetPendingItemIndex());
   EXPECT_TRUE(navigation_manager()->CanGoToOffset(-1));
   EXPECT_EQ(4, navigation_manager()->GetIndexForOffset(-1));
   EXPECT_TRUE(navigation_manager()->CanGoToOffset(-2));
@@ -485,4 +487,45 @@ TEST_F(NavigationManagerTest, OffsetsWithoutPendingIndex) {
   EXPECT_EQ(1000000004, navigation_manager()->GetIndexForOffset(1000000000));
 }
 
+// Tests offsets with pending transient entries (specifically gong back and
+// forward from a pending navigation entry that is added to the middle of the
+// navigation stack).
+TEST_F(NavigationManagerTest, OffsetsWithPendingTransientEntry) {
+  // Create a transient item in the middle of the navigation stack and go back
+  // to it (pending index is 1, current index is 2).
+  [session_controller() addPendingEntry:GURL("http://www.url.com/0")
+                               referrer:Referrer()
+                             transition:ui::PAGE_TRANSITION_LINK
+                      rendererInitiated:NO];
+  [session_controller() commitPendingEntry];
+  [session_controller() addPendingEntry:GURL("http://www.url.com/1")
+                               referrer:Referrer()
+                             transition:ui::PAGE_TRANSITION_LINK
+                      rendererInitiated:NO];
+  [session_controller() commitPendingEntry];
+  [session_controller() addPendingEntry:GURL("http://www.url.com/2")
+                               referrer:Referrer()
+                             transition:ui::PAGE_TRANSITION_LINK
+                      rendererInitiated:NO];
+  [session_controller() commitPendingEntry];
+  [session_controller() addTransientEntryWithURL:GURL("http://www.url.com/1")];
+  [session_controller() setPendingEntryIndex:1];
+
+  ASSERT_EQ(3, navigation_manager()->GetItemCount());
+  ASSERT_EQ(2, navigation_manager()->GetCurrentItemIndex());
+  ASSERT_EQ(1, navigation_manager()->GetPendingItemIndex());
+  EXPECT_EQ(2, navigation_manager()->GetIndexForOffset(1));
+  EXPECT_EQ(0, navigation_manager()->GetIndexForOffset(-1));
+
+  // Now go forward to that middle transient item (pending index is 1,
+  // current index is 0).
+  [session_controller() goToEntryAtIndex:0];
+  [session_controller() setPendingEntryIndex:1];
+  ASSERT_EQ(3, navigation_manager()->GetItemCount());
+  ASSERT_EQ(0, navigation_manager()->GetCurrentItemIndex());
+  ASSERT_EQ(1, navigation_manager()->GetPendingItemIndex());
+  EXPECT_EQ(2, navigation_manager()->GetIndexForOffset(1));
+  EXPECT_EQ(0, navigation_manager()->GetIndexForOffset(-1));
+}
+
 }  // namespace web