Returning correct duration estimate on Opus DTX packets.

Bug 4985 revealed two flaws
1. Opus duration estimate did not return correct length for DTX packets,

2. NetEq DoCodecInternalCng did not assign enough buffer.

P.S.
Generalizing problem 1, current NetEq decode function checks memory size by calling the duration estimate function. This is not ideal. A better way is to let codec's decode function to receive buffer size and return failure if it is not enough. This can be made in a separate CL.

BUG=webrtc:4985
R=henrik.lundin@webrtc.org

Review URL: https://codereview.webrtc.org/1334303005 .

Cr-Commit-Position: refs/heads/master@{#10031}

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

@@ -294,6 +294,20 @@ int WebRtcOpus_DurationEst(OpusDecInst* inst,
                            const uint8_t* payload,
                            size_t payload_length_bytes);
 
+/****************************************************************************
+ * WebRtcOpus_PlcDuration(...)
+ *
+ * This function calculates the duration of a frame returned by packet loss
+ * concealment (PLC).
+ *
+ * Input:
+ *        - inst                 : Decoder context
+ *
+ * Return value                  : The duration of a frame returned by PLC, in
+ *                                 samples per channel.
+ */
+int WebRtcOpus_PlcDuration(OpusDecInst* inst);
+
 /* TODO(minyue): Check whether it is needed to add a decoder context to the
  * arguments, like WebRtcOpus_DurationEst(...). In fact, the packet itself tells
  * the duration. The decoder context in WebRtcOpus_DurationEst(...) is not used.