[Android] Enable WebAudio on android x86 platform by default

In fact, functionality of WebAudio API was enabled on android x86 platform,
but it was disabled by default on android x86 chromium.

WebAudio is an important HTML5 API for Crosswalk, since some game engines
are using it, so we need to enable it by default.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1055
Merge pull request #1643 from yinli/webaudio-enable

[Android] Enable WebAudio on android x86 platform by default