[ios] Improved GetIndexForOffset performance to large offsets.

ios/web/navigation/navigation_manager_impl.mm

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/web/navigation/navigation_manager_impl.h"
 
 #include <stddef.h>
 
 #include <utility>
 
@@ skipping 332 matching lines @@
                    ? GetCurrentItemIndex()
                    : static_cast<int>([session_controller_ pendingEntryIndex]);
 
   if (offset < 0) {
     if (GetTransientItem()) {
       // Going back from transient entry is a matter of discarding it and there
       // is no need to move navigation index back.
       offset++;
     }
 
-    while (offset < 0) {
+    while (offset < 0 && result > 0) {
       // To stop the user getting 'stuck' on redirecting pages they weren't
       // even aware existed, it is necessary to pass over pages that would
       // immediately result in a redirect (the entry *before* the redirected
       // page).
       while (result > 0 && IsRedirectItemAtIndex(result)) {
         --result;
       }
       --result;
       ++offset;
     }
+    // Result may be out of bounds, so stop trying to skip redirect items and
+    // simply add the remainder.
+    result += offset;
+    if (result > GetItemCount() /* overflow */)
+      result = INT_MIN;
   } else if (offset > 0) {
     if (GetPendingItem() && [session_controller_ pendingEntryIndex] == -1) {
       // Chrome for iOS does not allow forward navigation if there is another
       // pending navigation in progress. Returning invalid index indicates that
       // forward navigation will not be allowed (and |INT_MAX| works for that).
       // This is different from other platforms which allow forward navigation
       // if pending entry exist.
       // TODO(crbug.com/661858): Remove this once back-forward navigation uses
       // pending index.
       return INT_MAX;
     }
-
-    while (offset > 0) {
+    while (offset > 0 && result < GetItemCount()) {
       ++result;
       --offset;
       // As with going back, skip over redirects.
       while (result + 1 < GetItemCount() && IsRedirectItemAtIndex(result + 1)) {
         ++result;
       }
     }
+    // Result may be out of bounds, so stop trying to skip redirect items and
+    // simply add the remainder.
+    result += offset;
+    if (result < 0 /* overflow */)
+      result = INT_MAX;
   }
 
   return result;
 }
 
 bool NavigationManagerImpl::IsRedirectItemAtIndex(int index) const {
   DCHECK_GT(index, 0);
   DCHECK_LT(index, GetItemCount());
   ui::PageTransition transition = GetItemAtIndex(index)->GetTransitionType();
   return transition & ui::PAGE_TRANSITION_IS_REDIRECT_MASK;
 }
 
 }  // namespace web