Enable leveldb build in GN world.

tools/gn/secondary/third_party/leveldatabase/BUILD.gn

 # Copyright (c) 2013 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.
 
 # Snappy is a compression library we use.
 # TODO(brettw) It's not clear why this needs to be parameterized.
 use_snappy = true
 
 defines = [ "LEVELDB_PLATFORM_CHROMIUM=1" ]
 
 config("leveldatabase_config") {
   include_dirs = [
     ".",
     "src",
     "src/include",
   ]
   if (is_win) {
     include_dirs += [ "src/port/win" ]
   }
 }
 
 static_library("leveldatabase") {
   sources = [
     "env_chromium.cc",
     "env_chromium.h",
+    "env_chromium_stdio.cc",
+    "env_chromium_stdio.h",
     "env_idb.h",
     "port/port_chromium.cc",
     "port/port_chromium.h",
     "src/db/builder.cc",
     "src/db/builder.h",
     "src/db/db_impl.cc",
     "src/db/db_impl.h",
     "src/db/db_iter.cc",
     "src/db/db_iter.h",
     "src/db/filename.cc",
@@ skipping 289 matching lines @@
 test("leveldb_write_batch_test") {
   sources = [
     "src/db/write_batch_test.cc",
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
   deps = [
     ":leveldb_testutil",
   ]
 }