Wrap ACM2 code inside acm2 namespace. This gurantees that one ACM would not use components of others by accident.

BUG=
R=minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2344004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4933 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_coding/main/acm2/acm_receiver.cc

@@ -30,6 +30,8 @@
 
 namespace webrtc {
 
+namespace acm2 {
+
 namespace {
 
 const int kNeteqInitSampleRateHz = 16000;
@@ -829,4 +831,6 @@ void AcmReceiver::InsertStreamOfSyncPackets(
   }
 }
 
+}  // namespace acm2
+
 }  // namespace webrtc