Add content renderer to the GN build

content/public/common/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("//content/common/common.gni")
 
 source_set("common") {
   sources = rebase_path(content_common_gypi_values.public_common_sources,
                         ".", "//content")
 
-  defines = [ "CONTENT_IMPLEMENTATION" ]
-
   configs += [
+    "//content:content_implementation",
     # TODO(GYP) remove when blink_headers target is converted.
     "//content:blink_headers_stub_config",
   ]
 
   deps = [
     "//content/common",
     "//skia",
     #"//third_party/WebKit/public:blink_headers",
   ]
 }