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 785 matching lines @@
 
 bool NavigationControllerImpl::RendererDidNavigate(
     RenderFrameHostImpl* rfh,
     const FrameHostMsg_DidCommitProvisionalLoad_Params& params,
     LoadCommittedDetails* details,
     bool is_navigation_within_page,
     NavigationHandleImpl* navigation_handle) {
   is_initial_navigation_ = false;
 
   // Save the previous state before we clobber it.
+  bool overriding_user_agent_changed = false;
   if (GetLastCommittedEntry()) {
     details->previous_url = GetLastCommittedEntry()->GetURL();
     details->previous_entry_index = GetLastCommittedEntryIndex();
+    if (pending_entry_ &&
+        pending_entry_->GetIsOverridingUserAgent() !=
+            GetLastCommittedEntry()->GetIsOverridingUserAgent())
+      overriding_user_agent_changed = true;
   } else {
     details->previous_url = GURL();
     details->previous_entry_index = -1;
   }
 
   // If there is a pending entry at this point, it should have a SiteInstance,
   // except for restored entries.
   DCHECK(pending_entry_index_ == -1 || pending_entry_->site_instance() ||
          pending_entry_->restore_type() != RestoreType::NONE);
   if (pending_entry_ && pending_entry_->restore_type() != RestoreType::NONE) {
@@ skipping 127 matching lines @@
   // we do not grant this entry additional bindings if we come back to it.
   active_entry->SetBindings(rfh->GetEnabledBindings());
 
   // Now prep the rest of the details for the notification and broadcast.
   details->entry = active_entry;
   details->is_main_frame = !rfh->GetParent();
   details->http_status_code = params.http_status_code;
 
   NotifyNavigationEntryCommitted(details);
 
+  if (overriding_user_agent_changed)
+    delegate_->UpdateOverridingUserAgent();

[nasko, 2017/01/09 23:07:53]

Technically this should only be called if we actually committed a new page,
right? Regardless whether it is in session history or not. It seems more
appropriate to put it in the cases we know it will happen than just generically
here.

[aelias_OOO_until_Jul13, 2017/01/09 23:10:52]

Right.  Could you give me a more concrete suggestion of where I should put this
instead?  This was the most specific place I found, but I'm not too familiar
with this code.

[nasko, 2017/01/09 23:44:03]

Yes. The switch statement on 850 calls a specific method for each type of
navigation. History navigations are classified as NAVIGATION_TYPE_EXISTING_PAGE,
so you can put the code inside of RendererDidNavigateToExistingPage. It is also
pretty helpful that this method enforces through DCHECK that it is only called
for main frames, which fits the behavior of our case too. 

[aelias_OOO_until_Jul13, 2017/01/10 01:17:43]

OK, done.  Note that I needed to change the underlying entry lookup to
GetActiveEntry() because GetVisibleEntry() only changed in
NotifyNavigationEntryCommitted().

+
   // Update the nav_entry_id for each RenderFrameHost in the tree, so that each
   // one knows the latest NavigationEntry it is showing (whether it has
   // committed anything in this navigation or not). This allows things like
   // state and title updates from RenderFrames to apply to the latest relevant
   // NavigationEntry.
   int nav_entry_id = active_entry->GetUniqueID();
   for (FrameTreeNode* node : delegate_->GetFrameTree()->Nodes())
     node->current_frame_host()->set_nav_entry_id(nav_entry_id);
   return true;
 }
@@ skipping 1187 matching lines @@
     }
   }
 }
 
 void NavigationControllerImpl::SetGetTimestampCallbackForTest(
     const base::Callback<base::Time()>& get_timestamp_callback) {
   get_timestamp_callback_ = get_timestamp_callback;
 }
 
 }  // namespace content