gn: Generalize process_version() and move it to build/util

chromecast/crypto/BUILD.gn

 # Copyright 2015 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("//chrome/version.gni")  # TODO layering violation!
 import("//testing/test.gni")
 
 source_set("crypto") {
   sources = [
     "signature_cache.cc",
     "signature_cache.h",
   ]
 
   deps = [
     "//base",
   ]
 }
 
 # Note: this doesn't correspond to a GYP unit test and should be merged with
 # others. (b/22952424)
 test("cast_crypto_unittests") {
   sources = [
     "signature_cache_unittest.cc",
   ]
 
   deps = [
     ":crypto",
     "//base",
     "//base/test:run_all_unittests",
     "//testing/gtest",
   ]
 }