Adding SetOpusMaxBandwidth in VoE and ACM

This is a step to solve
https://code.google.com/p/webrtc/issues/detail?id=1906

In particular, we add an API in VoE and ACM to call Opus's API of setting maximum bandwidth.

TEST = added a test in voe_cmd_test and listened to the result

BUG=
R=henrika@google.com, henrika@webrtc.org, turaj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/21129004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6869 4adac7df-926f-26a2-2b94-8c16560cd09d

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

@@ -261,6 +261,11 @@ int ACMOpus::SetPacketLossRate(int loss_rate) {
   return -1;
 }
 
+int ACMOpus::SetOpusMaxBandwidth(int max_bandwidth) {
+  // Ask the encoder to change the maximum required bandwidth.
+  return WebRtcOpus_SetMaxBandwidth(encoder_inst_ptr_, max_bandwidth);
+}
+
 #endif  // WEBRTC_CODEC_OPUS
 
 }  // namespace acm2