Fix mobile user agent string not being used when PlzNavigate is enabled.

content/browser/frame_host/navigation_request.cc

 // Copyright 2014 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 "content/browser/frame_host/navigation_request.h"
 
 #include <utility>
 
 #include "base/memory/ptr_util.h"
 #include "content/browser/appcache/appcache_navigation_handle.h"
@@ skipping 323 matching lines @@
     source_site_instance_ =
         frame_tree_node->current_frame_host()->GetSiteInstance();
   }
 
   // Update the load flags with cache information.
   UpdateLoadFlagsWithCacheFlags(&begin_params_.load_flags,
                                 common_params_.navigation_type,
                                 common_params_.method == "POST");
 
   // Add necessary headers that may not be present in the BeginNavigationParams.
-  const std::string user_agent_override =
-      frame_tree_node_->navigator()->GetDelegate()->GetUserAgentOverride();
+  std::string user_agent_override;
+  if (entry && entry->GetIsOverridingUserAgent()) {
+    user_agent_override =
+        frame_tree_node_->navigator()->GetDelegate()->GetUserAgentOverride();
+  }
+
   net::HttpRequestHeaders headers;
   headers.AddHeadersFromString(begin_params_.headers);
   AddAdditionalRequestHeaders(
       &headers, common_params_.url, common_params_.navigation_type,
       frame_tree_node_->navigator()->GetController()->GetBrowserContext(),
       common_params.method, user_agent_override, frame_tree_node);
   begin_params_.headers = headers.ToString();
 }
 
 NavigationRequest::~NavigationRequest() {
@@ skipping 477 matching lines @@
   DCHECK_EQ(request_params_.has_user_gesture, begin_params_.has_user_gesture);
 
   render_frame_host->CommitNavigation(response_.get(), std::move(body_),
                                       common_params_, request_params_,
                                       is_view_source_);
 
   frame_tree_node_->ResetNavigationRequest(true, true);
 }
 
 }  // namespace content