LibYuv GN: Add GN test targets.

libyuv.gni

+# Copyright 2016 The LibYuv Project Authors. All rights reserved.
+#
+# Use of this source code is governed by a BSD-style license
+# that can be found in the LICENSE file in the root of the source
+# tree. An additional intellectual property rights grant can be found
+# in the file PATENTS. All contributing project authors may
+# be found in the AUTHORS file in the root of the source tree.
+
+import("//build_overrides/build.gni")
+import("//build/config/arm.gni")
+
+declare_args() {
+  libyuv_include_tests = !build_with_chromium
+  libyuv_disable_jpeg = false
+  libyuv_use_neon = (current_cpu == "arm64" ||
+      (current_cpu == "arm" && (arm_use_neon || arm_optionally_use_neon)))
+}