Attempt to fix AOSP build fix

r168704 seems to break AOSP build which don't ENABLE_WEBAUDIO.

BUG=none
R=acolwell@chromium.org
TBR=abarth

Review URL: https://codereview.chromium.org/190973006

git-svn-id: svn://svn.chromium.org/blink/trunk@168755 bbb929c8-8fbe-4397-9dbb-9b2b20218538