Move gn secondary_source to the build/ directory

build/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" ]
 
@@ skipping 318 matching lines @@
     sources = [
       "src/db/write_batch_test.cc",
     ]
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
     deps = [
       ":leveldb_testutil",
     ]
   }
 }