Cleanup kUpdateSessionHistoryIndexBeforeNavigationStateChanged flag.

This rolled out successfully in r1139859 (M115) and the kill switch is
no longer needed.

Bug: 1439948
Change-Id: If893bcb300585235104f481f653060082b5c83c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4906760
Commit-Queue: Charlie Reis <creis@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1204666}

content/browser/renderer_host/navigation_controller_impl.cc

@@ -122,12 +122,6 @@
 namespace content {
 namespace {
 
-// TODO(https://crbug.com/1439948): Remove this base::Feature kill switch once
-// the feature safely rolls out.
-BASE_FEATURE(kUpdateSessionHistoryIndexBeforeNavigationStateChanged,
-             "UpdateSessionHistoryIndexBeforeNavigationStateChanged",
-             base::FEATURE_ENABLED_BY_DEFAULT);
-
 // Invoked when entries have been pruned, or removed. For example, if the
 // current entries are [google, digg, yahoo], with the current entry google,
 // and the user types in cnet, then digg and yahoo are pruned.
@@ -2218,10 +2212,7 @@ void NavigationControllerImpl::RendererDidNavigateToExistingEntry(
   // delegate sees the correct committed index when notified of navigation
   // state changes. (Otherwise CanGoBack may incorrectly return true, as in
   // https://crbug.com/1439948.)
-  if (base::FeatureList::IsEnabled(
-          kUpdateSessionHistoryIndexBeforeNavigationStateChanged)) {
-    last_committed_entry_index_ = GetIndexOfEntry(entry);
-  }
+  last_committed_entry_index_ = GetIndexOfEntry(entry);
 
   // We should also usually discard the pending entry if it corresponds to a
   // different navigation, since that one is now likely canceled.  In rare
@@ -2233,13 +2224,6 @@ void NavigationControllerImpl::RendererDidNavigateToExistingEntry(
   // actually change any other state, just kill the pointer.
   if (!keep_pending_entry)
     DiscardNonCommittedEntriesWithCommitDetails(commit_details);
-
-  if (!base::FeatureList::IsEnabled(
-          kUpdateSessionHistoryIndexBeforeNavigationStateChanged)) {
-    // Update the last committed index to reflect the committed entry.
-    // (This is legacy behavior, in case the kill-switch needs to be used.)
-    last_committed_entry_index_ = GetIndexOfEntry(entry);
-  }
 }
 
 void NavigationControllerImpl::RendererDidNavigateNewSubframe(