Extensions: Do not depend on device_serial_mojo target when extensions are turned off.

extensions/BUILD.gn

 # 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.
 
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 
 # GYP version: extensions/extensions_resources.gyp:extensions_resources
 group("extensions_resources") {
   deps = [
@@ skipping 14 matching lines @@
 
 # GYP version: extensions/extensions_resources.gyp:extensions_resources
 #              (exntensions_renderer_resources action)
 grit("extensions_renderer_resources") {
   source = "renderer/resources/extensions_renderer_resources.grd"
   outputs = [
     "grit/extensions_renderer_resources.h",
     "extensions_renderer_resources.pak",
   ]
   grit_flags = [ "-E", "mojom_root=" + rebase_path(root_gen_dir) ]
-  deps = [
-    "//device/serial:serial_mojo",
-  ]
+  if (enable_extensions) {
+    deps = [
+      "//device/serial:serial_mojo",
+    ]
+  }
 }
 
 source_set("test_support") {
   sources = [
     "browser/api/dns/mock_host_resolver_creator.cc",
     "browser/api/dns/mock_host_resolver_creator.h",
     "browser/api_test_utils.cc",
     "browser/api_test_utils.h",
     "browser/extensions_test.cc",
     "browser/extensions_test.h",
@@ skipping 129 matching lines @@
     "//testing/gtest",
   ]
 
   if (is_win) {
     deps += [
       "//base/allocator",
     ]
   }
 }
 }