Oilpan: Add static_cast to adjustAndMark implementation to assert that USING_GARBAGE_COLLECTED_MIXIN is given a correct type.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/298833002

git-svn-id: svn://svn.chromium.org/blink/trunk@174565 bbb929c8-8fbe-4397-9dbb-9b2b20218538