Add chrome test support target to GN build.

mojo/embedder/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.
 
 source_set("embedder") {
-  # This isn't really a standalone target, it must be linked into the

[jamesr, 2014/09/11 22:56:34]

why'd you delete this? it's still true

-  # mojo_system_impl component.
-  visibility = [ "//mojo/system" ]
-
   deps = [ "//base", ]
 
   defines = [
     "MOJO_SYSTEM_IMPL_IMPLEMENTATION",
     "MOJO_SYSTEM_IMPLEMENTATION",
   ]
 
   configs += [ "//mojo/system:system_config" ]
 
   sources = [
@@ skipping 42 matching lines @@
     "//mojo/system",
     "//testing/gtest",
   ]
 
   sources = [
     "embedder_unittest.cc",
     "platform_channel_pair_posix_unittest.cc",
     "simple_platform_shared_buffer_unittest.cc",
   ]
 }