Fixes compile and gn-check errors with enable_plugins=false.

content/ppapi_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("//ppapi/features/features.gni")
+
+assert(enable_plugins, "PPAPI plugins must be enabled.")
+
 group("ppapi_plugin") {
   visibility = [ "//content/*" ]  # This is an internal content API.
 
   if (is_component_build) {
     public_deps = [
       "//content",
     ]
   } else {
     public_deps = [
       ":ppapi_plugin_sources",
@@ skipping 49 matching lines @@
     "//third_party/icu",
     "//ui/base",
     "//ui/gfx",
     "//ui/gfx/geometry",
   ]
 
   if (is_win) {
     deps += [ "//ui/display" ]
   }
 }