Add most missing blink files to .gn files

third_party/WebKit/Source/core/clipboard/BUILD.gn

 # 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.
 
 import("//third_party/WebKit/Source/core/core.gni")
 
 blink_core_sources("clipboard") {
   sources = [
     "DataObject.cpp",
     "DataObject.h",
     "DataObjectItem.cpp",
     "DataObjectItem.h",
     "DataTransfer.cpp",
     "DataTransfer.h",
+    "DataTransferAccessPolicy.h",
     "DataTransferItem.cpp",
     "DataTransferItem.h",
     "DataTransferItemList.cpp",
     "DataTransferItemList.h",
     "DraggedIsolatedFileSystem.cpp",
     "DraggedIsolatedFileSystem.h",
     "Pasteboard.cpp",
     "Pasteboard.h",
   ]
 
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 }