Update gn files to support Mozilla build

Bug: webrtc:8670
No-Presubmit: true
Change-Id: I085dc63daa8274b5068540cbf56b6330f40643fa
Reviewed-on: https://webrtc-review.googlesource.com/38920
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21624}
diff --git a/modules/audio_coding/BUILD.gn b/modules/audio_coding/BUILD.gn
index f9eea0c..cc5036a 100644
--- a/modules/audio_coding/BUILD.gn
+++ b/modules/audio_coding/BUILD.gn
@@ -9,7 +9,9 @@
 import("../../webrtc.gni")
 import("audio_coding.gni")
 import("//build/config/arm.gni")
-import("//third_party/protobuf/proto_library.gni")
+if (!build_with_mozilla) {
+  import("//third_party/protobuf/proto_library.gni")
+}
 
 visibility = [ ":*" ]
 
@@ -853,7 +855,7 @@
   if (rtc_build_opus) {
     public_deps += [ rtc_opus_dir ]
   } else if (build_with_mozilla) {
-    include_dirs = [ getenv("DIST") + "/include/opus" ]
+    include_dirs = [ "/media/libopus/include" ]
   }
 }