pdf: Create a separate component for using the pdf pepper plugin.

components/pdf/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")
+
 static_library("common") {
   sources = [
-    "cdm_message_generator.cc",
-    "cdm_message_generator.h",
-    "cdm_messages_android.h",
+    "pdf_message_generator.cc",
+    "pdf_message_generator.h",
+    "pdf_messages.h",
   ]
 
   deps = [
     "//base",
     "//content/public/common",
     "//ipc",
+    "//url",
   ]
 }