Adding third_party/openh264/src, build files and unittests

third_party/openh264/testing/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("//testing/test.gni")
 
 test("openh264_unittests") {
   include_dirs = [
     "//",
     "//third_party",
   ]
 
   deps = [
     "//testing/gtest",
   ]
 
   sources = [
     "openh264_unittests.cc",
   ]
 }