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

BUG=none
TEST=none
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[email protected]
NOPRESUBMIT=true
NOTRY=true

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

Cr-Commit-Position: refs/heads/master@{#296141}
diff --git a/third_party/decklink/decklink.gyp b/third_party/decklink/decklink.gyp
index a9669b2..30772c48f 100644
--- a/third_party/decklink/decklink.gyp
+++ b/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"', {