Obviously, WebAudio is a core HTML5 API for crosswalk, it is important for us.
According to xingnan's @xingnan words, upstream guy intents to enable it by default on x86 platform in future, but there is no specific time, and I don't want to wait for upstream's patch and do a headache backport.
And this PR won't block anything, whatever it will be enabled or not by default in the future.
In addition, our QA can verify the WebAudio features more earlier.
So I suggest merge it now.