Do not zero-initialize RawPtr.

RawPtr is a transition type that will be removed and replace with a
normal pointer. Normal pointers are not default initialized and therefore
RawPtr shouldn't be either. Instead, we introduce a zap value in debug
mode to make it easier to find uninitialized RawPtrs during the transition.

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

Review URL: https://codereview.chromium.org/209433004

git-svn-id: svn://svn.chromium.org/blink/trunk@169922 bbb929c8-8fbe-4397-9dbb-9b2b20218538