Oilpan: Remove clang method export hack that is not longer needed.

The hack breaks the build on clang/windows and is apparently no longer
needed on clang/linux.

R=erik.corry@gmail.com, hans@chromium.org, oilpan-reviews@chromium.org, zerny@chromium.org

Review URL: https://codereview.chromium.org/353463002

git-svn-id: svn://svn.chromium.org/blink/trunk@176739 bbb929c8-8fbe-4397-9dbb-9b2b20218538