Enable the mojo-based serial API in the renderer behind a flag.

chrome/browser/extensions/chrome_content_browser_client_extensions_part.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 "chrome/browser/extensions/chrome_content_browser_client_extensions_part.h"
 
 #include <set>
 
 #include "base/command_line.h"
 #include "chrome/browser/browser_process.h"
@@ skipping 514 matching lines @@
                                            content::RenderProcessHost* process,
                                            Profile* profile) {
   if (!process)
     return;
   DCHECK(profile);
   if (ProcessMap::Get(profile)->Contains(process->GetID())) {
     command_line->AppendSwitch(switches::kExtensionProcess);
 #if defined(ENABLE_WEBRTC)
     command_line->AppendSwitch(::switches::kEnableWebRtcHWH264Encoding);
 #endif
+    if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+            switches::kEnableMojoSerialService)) {
+      command_line->AppendSwitch(switches::kEnableMojoSerialService);
+    }
   }
 }
 
 }  // namespace extensions