Don't overwrite WebRTC AEC dump file.

content/browser/resources/media/dump_creator.js

Index: content/browser/resources/media/dump_creator.js
diff --git a/content/browser/resources/media/dump_creator.js b/content/browser/resources/media/dump_creator.js
index a6c8dda14f8751be83d52f0f2a5dd1601020ad6a..33abe57e1e845a4463b479bf82a302170901215e 100644
--- a/content/browser/resources/media/dump_creator.js
+++ b/content/browser/resources/media/dump_creator.js
@@ -42,7 +42,11 @@ var DumpCreator = (function() {
         ' 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));