Add missing dependencies to the GN build.

content/public/browser/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("//content/browser/browser.gni")
 import("//build/config/ui.gni")
 
 # See //content/BUILD.gn for how this works.
 group("browser") {
   if (is_component_build) {
@@ skipping 25 matching lines @@
 
   if (use_aura) {
     sources -= [ "context_factory.h" ]
   }
 
   configs += [ "//content:content_implementation" ]
 
   public_deps = [
     # We expose skia headers in the public API.
     "//skia",
+    "//mojo/public/cpp/system",
   ]
   deps = [
     "//content/browser",
     "//content/public/common:common_sources",
     "//net",
     "//ui/accessibility",
     "//ui/base",
     "//ui/events",
   ]
 
   allow_circular_includes_from = [
     # This target is a pair with content/browser. They always go together and
     # include headers from each other.
     "//content/browser",
   ]
 }