fix(dtmf): enqueue tones to be played (#994)

Calls to insertDTMF will stop any tone
playing in progress. Prevent such by
checking the tonebuffer for any tones
in play and queue tone playing if
any tones are in play.

modules/RTC/TraceablePeerConnection.js

      */
     this._dtmfSender = undefined;
 
+    /**
+     * @typedef {Object} TouchToneRequest
+     * @property {string} tones - The DTMF tones string as defined by
+     * {@code RTCDTMFSender.insertDTMF}, 'tones' argument.
+     * @property {number} duration - The amount of time in milliseconds that
+     * each DTMF should last.
+     * @property {string} interToneGap - The length of time in miliseconds to
+     * wait between tones.
+     */
+    /**
+     * TouchToneRequests which are waiting to be played. This queue is filled
+     * if there are touch tones currently being played.
+     *
+     * @type {Array<TouchToneRequest>}
+     * @private
+     */
+    this._dtmfTonesQueue = [];
+
     /**
      * Indicates whether or not this peer connection instance is actively
      * sending/receiving video media. When set to <tt>false</tt> the SDP video
             }
             this._dtmfSender && logger.info(`${this} initialized DTMFSender using deprecated createDTMFSender`);
         }
+
+        if (this._dtmfSender) {
+            this._dtmfSender.ontonechange = this._onToneChange.bind(this);
+        }
     }
 
     if (this._dtmfSender) {
+        if (this._dtmfSender.toneBuffer) {
+            this._dtmfTonesQueue.push({
+                tones,
+                duration,
+                interToneGap
+            });
+
+            return;
+        }
+
         this._dtmfSender.insertDTMF(tones, duration, interToneGap);
     } else {
         logger.warn(`${this} sendTones - failed to select DTMFSender`);
     }
 };
 
+/**
+ * Callback ivoked by {@code this._dtmfSender} when it has finished playing
+ * a single tone.
+ *
+ * @param {Object} event - The tonechange event which indicates what characters
+ * are left to be played for the current tone.
+ * @private
+ * @returns {void}
+ */
+TraceablePeerConnection.prototype._onToneChange = function(event) {
+    // An empty event.tone indicates the current tones have finished playing.
+    // Automatically start playing any queued tones on finish.
+    if (this._dtmfSender && event.tone === '' && this._dtmfTonesQueue.length) {
+        const { tones, duration, interToneGap } = this._dtmfTonesQueue.shift();
+
+        this._dtmfSender.insertDTMF(tones, duration, interToneGap);
+    }
+};
+
 /**
  * Makes the underlying TraceablePeerConnection generate new SSRC for
  * the recvonly video stream.
 
     this._addedStreams = [];
 
+    this._dtmfSender = null;
+    this._dtmfTonesQueue = [];
+
     if (!this.rtc._removePeerConnection(this)) {
         logger.error('RTC._removePeerConnection returned false');
     }