Make chrome GN build work in component mode.

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("//build/config/features.gni")
 import("//content/common/common.gni")
 
-source_set("common") {
+# See //content/BUILD.gn for how this works.
+group("common") {
+  if (is_component_build) {
+    deps = [ "//content" ]
+  } else {
+    deps = [ ":common_sources" ]
+  }
+  forward_dependent_configs_from = deps
+}
+
+source_set("common_sources") {
+  visibility = [ "//content/*" ]
+
   sources = rebase_path(content_common_gypi_values.public_common_sources,
                         ".", "//content")
 
   configs += [
     "//content:content_implementation",
   ]
 
   deps = [
     "//content/common",
     "//net",
     "//skia",
     "//third_party/WebKit/public:blink_headers",
     "//third_party/icu",
     "//ui/base",
     "//ui/gfx",
   ]
 
   forward_dependent_configs_from = [ "//content/common" ]
 
   if (!enable_plugins) {
     sources -= [
       "pepper_plugin_info.cc",
       "pepper_plugin_info.h",
     ]
   }
 }