Bail out in forceIsac16KInSdp if there is no audio content in the SDP.

BUG=

Review URL: https://codereview.chromium.org/554753003

Cr-Commit-Position: refs/heads/master@{#293971}

content/test/data/media/peerconnection-call.html

@@ -27,11 +27,15 @@
   var maybeForceIsac16K = function(sdp) { return sdp; };
   function forceIsac16KInSdp() {
     maybeForceIsac16K = function(sdp) {
+      if (sdp.search('m=audio') == -1)
+        return sdp;
+
       sdp = sdp.replace(/m=audio (\d+) RTP\/SAVPF.*\r\n/g,
                         'm=audio $1 RTP/SAVPF 103 126\r\n');
       sdp = sdp.replace('a=fmtp:111 minptime=10', 'a=fmtp:103 minptime=10');
       if (sdp.search('a=rtpmap:103 ISAC/16000') == -1)
         failTest('Missing iSAC 16K codec on Android; cannot force codec.');
+
       return sdp;
     };
     sendValueToTest('isac-forced');