Fix Gn untracked headers in webrtc/media

BUG=webrtc:7645

Review-Url: https://codereview.webrtc.org/3013063002
Review-Url: https://codereview.webrtc.org/3018473002
Cr-Original-Commit-Position: refs/heads/master@{#19844}
Cr-Original-Original-Commit-Position: refs/heads/master@{#19824}
Cr-Mirrored-From: https://webrtc.googlesource.com/src
Cr-Mirrored-Commit: 6674846b4adc999f69bfdb12080749d4e4ab729d
diff --git a/media/BUILD.gn b/media/BUILD.gn
index 9deddee..26d6625 100644
--- a/media/BUILD.gn
+++ b/media/BUILD.gn
@@ -140,6 +140,7 @@
     "engine/adm_helpers.h",
     "engine/apm_helpers.cc",
     "engine/apm_helpers.h",
+    "engine/constants.h",
     "engine/internaldecoderfactory.cc",
     "engine/internaldecoderfactory.h",
     "engine/internalencoderfactory.cc",