Opus mono/stereo on the same payloadtype, and fix of memory bug

During call setup Opus should always be signaled as a 48000 Hz stereo codec, not depending on what we plan to send, or how we plan to decode received packets.
The previous implementation had different payload types for mono and stereo, which breaks the proposed standard.

While working on this CL I ran in to the problem reported earlier, that we could get a crash related to deleting decoder memory. This should now be solved in Patch Set 3.

BUG=issue1013, issue1112

Review URL: https://webrtc-codereview.appspot.com/933022

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3177 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_coding/codecs/opus/interface/opus_interface.h

@@ -62,6 +62,13 @@ int16_t WebRtcOpus_SetBitRate(OpusEncInst* inst, int32_t rate);
 int16_t WebRtcOpus_DecoderCreate(OpusDecInst** inst, int channels);
 int16_t WebRtcOpus_DecoderFree(OpusDecInst* inst);
 
+/****************************************************************************
+ * WebRtcOpus_DecoderChannels(...)
+ *
+ * This function returns the number of channels created for Opus decoder.
+ */
+int WebRtcOpus_DecoderChannels(OpusDecInst* inst);
+
 /****************************************************************************
  * WebRtcOpus_DecoderInit(...)
  *