[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}

diff --git a/third_party/decklink/BUILD.gn b/third_party/decklink/BUILD.gn
new file mode 100644
index 0000000..6847d9c
--- /dev/null
+++ b/third_party/decklink/BUILD.gn

@@ -0,0 +1,12 @@
+# 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",
+    ]
+  }
+}