Add GN targets for mojom generator

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 14 matching lines @@
     "//components/os_crypt",
     "//components/startup_metric_utils",
     "//components/resources:components_resources",
     "//components/translate:translate_core_browser",
     "//components/translate:translate_core_common",
     "//components/url_matcher",
     "//crypto",
     "//device/usb",
     #"//extensions/common/api:extensions_api",
     "//ipc",
+    "//mojo",
     "//net",
     #"//sdch",
     "//skia",
     #"//third_party/WebKit/Source/platform",
     "//third_party/icu",
     "//third_party/leveldatabase",
     "//third_party/libpng",
     "//third_party/libusb",
     "//third_party/libwebp",
     "//third_party/re2",
@@ skipping 24 matching lines @@
       "//net",
       "//skia",
       "//third_party/libusb",
       "//third_party/WebKit/Source/wtf",  # TODO(brettw) re-enable for Android.
       "//tools/gn",
       "//ui/events",
       "//ui/gfx",
     ]
   }
 }