Oilpan: Fix build broken by r175995.

TypeConversions class needs to be GarbageCollectedFinalized because new String
member needs finalization.

BUG=
TBR=oilpan-reviews@chromium.org

Review URL: https://codereview.chromium.org/326423006

git-svn-id: svn://svn.chromium.org/blink/trunk@176008 bbb929c8-8fbe-4397-9dbb-9b2b20218538