Add missing dependencies to the GN build.

content/child/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("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//content/child/child.gni")
 
 source_set("child") {
   # Only the public target should depend on this. All other targets (even
   # internal content ones) should depend on the public one.
   visibility = [ "//content/public/child:child_sources" ]
 
   sources = rebase_path(content_child_gypi_values.private_child_sources,
                         ".", "//content")
 
+  public_deps = [
+    "//mojo/edk/system",
+  ]
+
   deps = [
     "//base",
     "//components/tracing",
     "//mojo/common",
     "//mojo/environment:chromium",
     "//mojo/public/interfaces/application",
     "//skia",
     "//third_party/icu",
     "//ui/base",
     "//ui/gfx",
@@ skipping 99 matching lines @@
   }
 
   if (!use_openssl) {
     sources += rebase_path(content_child_gypi_values.webcrypto_nss_sources,
                            ".", "//content")
   } else {
     sources += rebase_path(content_child_gypi_values.webcrypto_openssl_sources,
                            ".", "//content")
   }
 }