Add GN Build file for extensions_common target.

BUILD.gn

 # Copyright (c) 2013 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.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 17 matching lines @@
     "//chrome/plugin",
     "//chrome/renderer",
     "//chrome/service",
     "//chrome/utility",
     "//components:all_components",
     "//content",
     "//crypto",
     "//device/bluetooth",
     "//device/nfc",
     "//device/usb",
+    "//extensions/common",
     "//extensions/common/api",
     "//gin",
     "//gpu",
     "//google_apis",
     "//google_apis/gcm",
     "//ipc",
     "//media",
     "//mojo",
     "//mojo/common",
     "//mojo/environment:chromium",
@@ skipping 109 matching lines @@
       "//chrome/browser",  # Blocked on content.
       "//chrome/browser/devtools",  # Blocked on content.
       "//chrome/browser/ui",  # Blocked on content.
       "//chrome/browser/ui/views",  # Blocked on content.
       "//chrome/common",  # Blocked on content.
       "//chrome/plugin",  # Blocked on content.
       "//chrome/renderer",  # Blocked on content.
       "//chrome/service",  # Blocked on content.
       "//chrome/utility",  # Blocked on content.
       "//content",
+      "//extensions/common",
       "//extensions/common/api",
       "//pdf",  # Not compiled on Android in GYP yet, either.
       "//ppapi:ppapi_c",
       "//sandbox",
       "//third_party/libusb",
       "//ui/keyboard",  # Blocked on content.
 
       # Seems to not be compiled on Android. Otherwise it will need a config.h.
       "//third_party/libxslt",
 
@@ skipping 25 matching lines @@
     ]
   }
 
   if (use_ozone) {
     deps += [
       "//ui/events/ozone:events_ozone",
       "//ui/events/ozone:events_ozone_evdev",
     ]
   }
 }