[ios] Improved GetIndexForOffset performance to large offsets.

ios/web/navigation/crw_session_controller.mm

 // Copyright 2012 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.
 
 #import "ios/web/navigation/crw_session_controller.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <utility>
@@ skipping 754 matching lines @@
 
 - (NSInteger)indexOfEntryForDelta:(int)delta {
   NSInteger result = _currentNavigationIndex;
   if (delta < 0) {
     if (_transientEntry) {
       // Going back from transient entry is a matter of discarding it and there
       // is no need to move navigation index back.
       delta++;
     }
 
-    while (delta < 0) {
+    while (delta < 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 && [self isRedirectTransitionForEntryAtIndex:result]) {
         --result;
       }
       --result;
       ++delta;
     }
+    // Result may be out of bounds, so stop trying to skip redirect items and
+    // simply add the remainder.
+    result += delta;
   } else if (delta > 0) {
     NSInteger count = static_cast<NSInteger>([_entries count]);
     if (_pendingEntry && _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 |NSNotFound| 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 NSNotFound;
     }
 
-    while (delta > 0) {
+    while (delta > 0 && result < count) {
       ++result;
       --delta;
       // As with going back, skip over redirects.
       while (result + 1 < count &&
              [self isRedirectTransitionForEntryAtIndex:result + 1]) {
         ++result;
       }
     }
+    // Result may be out of bounds, so stop trying to skip redirect items and
+    // simply add the remainder.
+    result += delta;
   }
 
   return result;
 }
 
 #pragma mark -
 #pragma mark Private methods
 
 - (NSString*)uniqueID {
   CFUUIDRef uuidRef = CFUUIDCreate(NULL);
@@ skipping 34 matching lines @@
   return [[CRWSessionEntry alloc] initWithNavigationItem:std::move(item)];
 }
 
 - (BOOL)isRedirectTransitionForEntryAtIndex:(NSInteger)index {
   ui::PageTransition transition =
       [_entries[index] navigationItem]->GetTransitionType();
   return (transition & ui::PAGE_TRANSITION_IS_REDIRECT_MASK) ? YES : NO;
 }
 
 @end