Remove unnecessary dependencies on mojo/edk/system

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.
 
 # TODO(rockot) bug 505926: Don't include chrome files from here.
 # See chrome_browser_tests_extensions_sources below
 import("//chrome/chrome_tests.gni")
 import("//extensions/extensions.gni")
 import("//testing/test.gni")
 import("//tools/grit/grit_rule.gni")
@@ skipping 223 matching lines @@
     "//base/test:test_support",
     "//components/prefs:test_support",
     "//components/storage_monitor:test_support",
     "//content/test:browsertest_base",
     "//content/test:test_support",
     "//device/bluetooth:mocks",
     "//device/core:mocks",
     "//device/hid:mocks",
     "//device/usb:test_support",
     "//mojo/edk/js",
-    "//mojo/edk/system",
     "//mojo/public/cpp/bindings",
     "//testing/gmock",
     "//testing/gtest",
   ]
 
   data_deps = [
     "//third_party/mesa:osmesa",
   ]
 
   if (is_mac) {
@@ skipping 105 matching lines @@
     "//testing/gtest",
     "//third_party/hunspell",
     "//third_party/icu",
     "//third_party/libpng",
     "//third_party/zlib",
     "//ui/base:test_support",
     "//ui/resources:ui_test_pak",
     "//ui/web_dialogs:test_support",
   ]
 }