Oilpan: Enable Oilpan by default for core/testing

core/testing is for testing only, so there would be no performance concern in shipping Oilpan.

BUG=340522

Review URL: https://codereview.chromium.org/591023002

git-svn-id: svn://svn.chromium.org/blink/trunk@182403 bbb929c8-8fbe-4397-9dbb-9b2b20218538