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

third_party/decklink/BUILD.gn

Index: third_party/decklink/BUILD.gn
diff --git a/third_party/libwebm/libwebm.gypi b/third_party/decklink/BUILD.gn
similarity index 49%
copy from third_party/libwebm/libwebm.gypi
copy to third_party/decklink/BUILD.gn
index 074110145273d07113fc3af4ea69c0e3438d958c..6847d9c24d8b7f94ddc6dcc181d6160ee844bded 100644
--- a/third_party/libwebm/libwebm.gypi
+++ b/third_party/decklink/BUILD.gn
@@ -2,12 +2,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-{
-  'variables': {
-    'libwebm_sources': [
-      'source/mkvmuxer.cpp',
-      'source/mkvmuxerutil.cpp',
-      'source/mkvwriter.cpp',
-    ],
+# GYP version: //third_party/decklink/decklink.gyp:decklink
+if (is_mac) {
+  static_library("decklink") {
+    sources = [
+      "mac/include/DeckLinkAPIDispatch.cpp",
+    ]
   }
 }