Only build libsrtp tests if a build flag is set

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("//testing/test.gni")
 
+declare_args() {
+  # Tests may not be appropriate for some build environments, e.g. Windows.
+  # Rather than enumerate valid options, we just let clients ask for them.
+  build_libsrtp_tests = false
+}
+
 config("libsrtp_config") {
   defines = [
     # Package identity
     "PACKAGE_STRING=\"libsrtp2 2.0.0-pre\"",
     "PACKAGE_VERSION=\"2.0.0-pre\"",
 
     # Config
     "HAVE_CONFIG_H",
     "OPENSSL",
 
@@ skipping 90 matching lines @@
     "crypto/replay/rdb.c",
     "crypto/replay/rdbx.c",
     "srtp/ekt.c",
     "srtp/srtp.c",
   ]
   public_deps = [
     "//third_party/boringssl:boringssl",
   ]
 }
 
-if (is_win) {
-  # Don't build tests on Windows. Upstream doesn't, so we can't assume the
-  # tests will compile or run.
-
-  group("srtp_tests") {

[mattdr-at-chromium, 2016/10/20 10:45:10]

I decided there wasn't value in providing an empty version of this target. I'm
just going to get rid of the reference in src/BUILD.gn.

-    testonly = true
-  }
-} else {
+if (build_libsrtp_tests) {
   config("srtp_test_config") {
     include_dirs = [ "test" ]
     configs = [ ":libsrtp_warnings" ]
   }
 
   template("srtp_test") {
     test("${target_name}") {
       output_name = invoker.target_name
       output_dir = "${root_out_dir}/srtp_tests"
       configs -= [ "//build/config/compiler:chromium_code" ]
@@ skipping 85 matching lines @@
       ":rdbx_driver",
       ":replay_driver",
       ":roc_driver",
       ":rtpw",
       ":srtp_driver",
       ":test_scripts",
     ]
     testonly = true
   }
 }