Add embedder api for participating in gpu logic

content/public/gpu/BUILD.gn

-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2016 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.
 
 # See //content/BUILD.gn for how this works.
-group("plugin") {
+group("gpu") {
   if (is_component_build) {
     public_deps = [
       "//content",
     ]
   } else {
     public_deps = [
-      ":plugin_sources",
+      ":gpu_sources",
     ]
   }
 }
 
-source_set("plugin_sources") {
+source_set("gpu_sources") {
   visibility = [ "//content/*" ]
 
   sources = [
-    "content_plugin_client.h",
+    "content_gpu_client.h",
   ]
 
   deps = [
     "//base",
-    "//content/plugin",
-    "//content/public/common:common_sources",
+    "//content:export",
+    "//content/gpu:gpu_sources",
   ]
 }