Fix Gn untracked headers in webrtc/media

BUG=webrtc:7645

Review-Url: https://codereview.webrtc.org/3013063002
Cr-Original-Commit-Position: refs/heads/master@{#19824}
Review-Url: https://codereview.webrtc.org/3018473002
Cr-Commit-Position: refs/heads/master@{#19844}

Author: charu0291-zz

webrtc/media/BUILD.gn

@@ -140,6 +140,7 @@ rtc_static_library("rtc_audio_video") {
     "engine/adm_helpers.h",
     "engine/apm_helpers.cc",
     "engine/apm_helpers.h",
+    "engine/constants.h",
     "engine/internaldecoderfactory.cc",
     "engine/internaldecoderfactory.h",
     "engine/internalencoderfactory.cc",