[misc-xwalk] Move WebAudio sample to xwalk system tests

- App crashes after some time because of Audio Decoder
- See https://crosswalk-project.org/jira/browse/XWALK-1993

Impacted TCs num(approved): New 0, Update 1, Delete 0
Unit test Platform: Android
Test result summary: Pass 0, Fail 1, Blocked 0
=> Click the "Automatic Audio Tag" to run the same for about 2 minutes.

Test step, I think, is a user action step; only focus on what to do.