Add DCHECKs for invariants on NavigationController indices.

content/browser/frame_host/navigation_controller_impl.cc

Index: content/browser/frame_host/navigation_controller_impl.cc
diff --git a/content/browser/frame_host/navigation_controller_impl.cc b/content/browser/frame_host/navigation_controller_impl.cc
index 768f595fac70a59363483b33f06821983b03fc60..fd0c5f527bdee4ebde7595b34a22e4b85573551c 100644
--- a/content/browser/frame_host/navigation_controller_impl.cc
+++ b/content/browser/frame_host/navigation_controller_impl.cc
@@ -534,11 +534,15 @@ bool NavigationControllerImpl::CanViewSource() const {
 }
 
 int NavigationControllerImpl::GetLastCommittedEntryIndex() const {
+  // The last committed entry index must always be less than the number of
+  // entries.  It should be -1 iff there are no entries.
+  DCHECK_LT(last_committed_entry_index_, GetEntryCount());
+  DCHECK_EQ(last_committed_entry_index_ == -1, GetEntryCount() == 0);
   return last_committed_entry_index_;
 }
 
 int NavigationControllerImpl::GetEntryCount() const {
-  DCHECK(entries_.size() <= max_entry_count());
+  DCHECK_LE(entries_.size(), max_entry_count());
   return static_cast<int>(entries_.size());
 }
 
@@ -1726,10 +1730,22 @@ void NavigationControllerImpl::DiscardNonCommittedEntries() {
 }
 
 NavigationEntryImpl* NavigationControllerImpl::GetPendingEntry() const {
+  // If there is no pending_entry_, there should be no pending_entry_index_.
+  DCHECK(pending_entry_ || pending_entry_index_ == -1);
+
+  // If there is a pending_entry_index_, then pending_entry_ must be the entry
+  // at that index.
+  DCHECK(pending_entry_index_ == -1 ||
+         pending_entry_ == GetEntryAtIndex(pending_entry_index_));
+
   return pending_entry_;
 }
 
 int NavigationControllerImpl::GetPendingEntryIndex() const {
+  // The pending entry index must always be less than the number of entries.
+  // If there are no entries, it must be exactly -1.
+  DCHECK_LT(pending_entry_index_, GetEntryCount());
+  DCHECK(GetEntryCount() != 0 || pending_entry_index_ == -1);
   return pending_entry_index_;
 }