Hide the symbols from the oilpan assembly files.

R=erik.corry@gmail.com, haraken@chromium.org, vegorov@chromium.org, wibling@chromium.org
BUG=

Review URL: https://codereview.chromium.org/140883002

git-svn-id: svn://svn.chromium.org/blink/trunk@165279 bbb929c8-8fbe-4397-9dbb-9b2b20218538