Make chrome GN build work in component mode.

content/plugin/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")
 
 # This is the NPAPI plugin process. It isn't used on Linux.
 if (enable_plugins && !is_linux) {
   source_set("plugin") {
-    visibility = [ "//content/*" ]
+    # Only the public target should depend on this. All other targets (even
+    # internal content ones) should depend on the public one.
+    visibility = [ "//content/public/plugin:plugin_sources" ]
+
     sources = [
       "plugin_channel.cc",
       "plugin_channel.h",
       "plugin_interpose_util_mac.mm",
       "plugin_interpose_util_mac.h",
       "plugin_main.cc",
       "plugin_main_mac.mm",
       "plugin_thread.cc",
       "plugin_thread.h",
       "webplugin_accelerated_surface_proxy_mac.cc",
@@ skipping 12 matching lines @@
       "//skia",
       "//third_party/npapi",
       "//third_party/WebKit/public:blink",
     ]
   }
 } else {
   # This way it can be unconditionally depended on.
   group("plugin") {
   }
 }