Update viewport_meta setting on any navigation.

content/browser/frame_host/navigation_controller_impl.cc

 // 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.
 
 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  *     (http://www.torchmobile.com/)
  *
@@ skipping 1201 matching lines @@
   } else if (params.nav_entry_id) {
     // This is a browser-initiated navigation (back/forward/reload).
     entry = GetEntryWithUniqueID(params.nav_entry_id);
 
     // Needed for the restore case, where the serialized NavigationEntry doesn't
     // have the SSL state. Note that for in-page navigation, there's no
     // SSLStatus in the NavigationHandle so don't overwrite the existing entry's
     // SSLStatus.
     if (!is_in_page)
       entry->GetSSL() = handle->ssl_status();
+
+    // Back/forward navigations can alter "request desktop site" state.
+    if (entry && GetLastCommittedEntry() &&
+        entry->GetIsOverridingUserAgent() !=
+            GetLastCommittedEntry()->GetIsOverridingUserAgent())
+      delegate_->UpdateOverridingUserAgent();
   } else {
     // This is renderer-initiated. The only kinds of renderer-initated
     // navigations that are EXISTING_PAGE are reloads and location.replace,
     // which land us at the last committed entry.
     entry = GetLastCommittedEntry();
 
     // If this is an in-page navigation, then there's no SSLStatus in the
     // NavigationHandle so don't overwrite the existing entry's SSLStatus.
     if (!is_in_page)
       entry->GetSSL() = handle->ssl_status();
@@ skipping 921 matching lines @@
     }
   }
 }
 
 void NavigationControllerImpl::SetGetTimestampCallbackForTest(
     const base::Callback<base::Time()>& get_timestamp_callback) {
   get_timestamp_callback_ = get_timestamp_callback;
 }
 
 }  // namespace content