Fix WebAudio crash caused by setClient() getting called on the wrong thread.

BUG=326337
TEST=Fix verified by LayoutTests/webaudio/mediaelementaudiosourcenode-play.html

Review URL: https://codereview.chromium.org/103633006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240785 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240785