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/voice_engine/voe_codec_impl.h

@@ -54,6 +54,8 @@ public:
                              VadModes& mode,
                              bool& disabledDTX);
 
+    virtual int SetOpusMaxBandwidth(int channel, int bandwidth_hz);
+
     // Dual-streaming
     virtual int SetSecondarySendCodec(int channel, const CodecInst& codec,
                                       int red_payload_type);