Don't overwrite WebRTC AEC dump file.

content/browser/resources/media/dump_creator.js

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 
 /**
  * Provides the UI for dump creation.
  */
 var DumpCreator = (function() {
   /**
@@ skipping 24 matching lines @@
         'Enable diagnostic audio recordings.</label></p>' +
         '<p>A diagnostic audio recording is used for analyzing audio' +
         ' problems. It contains the audio played out from the speaker and' +
         ' recorded from the microphone and is saved to the local disk.' +
         ' Checking this box will enable the recording for an ongoing WebRTC' +
         ' call and for future WebRTC calls. When the box is unchecked or this' +
         ' page is closed, this recording functionality will be disabled for' +
         ' future WebRTC calls, but an ongoing call will continue to record' +
         ' until the call is ended. Only recording in one tab is supported.' +
         ' If several tabs are running WebRTC calls, the resulting file will' +
-        ' be invalid.</p>';
+        ' be invalid. To restart the dump, the tab with the call being' +
+        ' recorded must be closed and recording disabled and enabled again.' +
+        ' When enabling, you select a file to save the dump to. Choose a' +
+        ' non-existing file name. Selecting an existing file will append to' +
+        ' it, not overwrite it, rendering the file invalid. </p>';
 
     content.getElementsByTagName('a')[0].addEventListener(
         'click', this.onDownloadData_.bind(this));
     content.getElementsByTagName('input')[0].addEventListener(
         'click', this.onAecRecordingChanged_.bind(this));
   }
 
   DumpCreator.prototype = {
     /** Mark the AEC recording checkbox checked.*/
     enableAecRecording: function() {
@@ skipping 28 matching lines @@
       var enabled = this.root_.getElementsByTagName('input')[0].checked;
       if (enabled) {
         chrome.send('enableAecRecording');
       } else {
         chrome.send('disableAecRecording');
       }
     },
   };
   return DumpCreator;
 })();