[GN build] Add a BUILD.gn file for //third_party/decklink

BUG=none
TEST=none
R=jamesr@chromium.org
TBR=perkj@chromium.org
NOPRESUBMIT=true
NOTRY=true

Review URL: https://codereview.chromium.org/594653004

Cr-Commit-Position: refs/heads/master@{#296141}

third_party/decklink/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.
+
+# GYP version: //third_party/decklink/decklink.gyp:decklink
+if (is_mac) {
+  static_library("decklink") {
+    sources = [
+      "mac/include/DeckLinkAPIDispatch.cpp",
+    ]
+  }
+}

third_party/decklink/decklink.gyp

@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+# GN version: //third_party/decklink
 {
   'conditions': [
     ['OS=="mac"', {