Oilpan: WebAudio: Speculative fix for a crash in AudioContext::uninitialize.

AudioContext object might be destructed between stop() and uninitialize(). This
CL ensures that |this| is alive by WTF::bind.

BUG=399110

Review URL: https://codereview.chromium.org/437773002

git-svn-id: svn://svn.chromium.org/blink/trunk@179425 bbb929c8-8fbe-4397-9dbb-9b2b20218538