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

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=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175277

[Mikhail, 2014-06-02 09:52:22]

lgtm

[Mads Ager (chromium), 2014-06-02 10:05:31]

lgtm2

[Erik Corry, 2014-06-02 10:25:55]

The CQ bit was checked by erik.corry@gmail.com

[commit-bot: I haz the power, 2014-06-02 10:26:11]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/erik.corry@gmail.com/308323002/1

[commit-bot: I haz the power, 2014-06-02 11:42:24]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_rel/builds/1...)

[commit-bot: I haz the power, 2014-06-02 12:05:19]

Change committed as 175277