Adding third_party/openh264/src, build files and unittests

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.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 55 matching lines @@
 group("both_gn_and_gyp") {
   testonly = true
   deps = [
     "//base:base_unittests",
     "//chrome/installer",
     "//components:components_unittests",
     "//net:net_unittests",
     "//skia:skia_unittests",
     "//sql:sql_unittests",
     "//sync:sync_unit_tests",
-    "//third_party/openh264/tests:openh264_unittests",
     "//ui/base:ui_base_unittests",
     "//ui/gfx:gfx_unittests",
     "//url:url_unittests",
   ]
 
   if (!is_ios && !is_android && !is_chromecast) {
     deps += [
       "//chrome",
       "//chrome/test:browser_tests",
       "//chrome/test:interactive_ui_tests",
@@ skipping 712 matching lines @@
       if (target_cpu == "x86") {
         deps += [
           # "//content/shell:crash_service_win64", TODO(GYP): crbug.com/537009
         ]
       }
     } else {
       deps += [ "//breakpad:minidump_stackwalk($host_toolchain)" ]
     }
   }
 }