Move const variables initialization to constants.cc

Bug: None
Change-Id: I73f0d343e242e959879c5396852270c78c17fc87
Reviewed-on: https://webrtc-review.googlesource.com/22420
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20828}
diff --git a/media/BUILD.gn b/media/BUILD.gn
index 5370791..4ff272f 100644
--- a/media/BUILD.gn
+++ b/media/BUILD.gn
@@ -143,6 +143,7 @@
     "engine/adm_helpers.h",
     "engine/apm_helpers.cc",
     "engine/apm_helpers.h",
+    "engine/constants.cc",
     "engine/constants.h",
     "engine/convert_legacy_video_factory.cc",
     "engine/convert_legacy_video_factory.h",