Oilpan: Add STACK_ALLOCATED to AutoLocker

This is a preparation for enabling Oilpan for webaudio/.

BUG=340522
NOTRY=true

Review URL: https://codereview.chromium.org/438313002

git-svn-id: svn://svn.chromium.org/blink/trunk@179549 bbb929c8-8fbe-4397-9dbb-9b2b20218538