Oilpan: Clear unused slots in Deque so they are not found in conservative GCs

If we find a direct pointer to the backing store of the deque (this will
normally be an iterator) we have to trace the backing without knowing how
much of it is in use (that information is in the Deque itself, not the
backing store).  This means we have to ensure that unused areas of the
backing store are zeroed out.
R=ager@chromium.org, mikhail.pozdnyakov@untel.com
BUG=

Review URL: https://codereview.chromium.org/308323002

git-svn-id: svn://svn.chromium.org/blink/trunk@175277 bbb929c8-8fbe-4397-9dbb-9b2b20218538