When reading the map from a live object, use a barrier load

When reading the map from a live object, use a barrier load

It could happen that we shrink a live object on the main thread (e.g.
MigrateFastToSlow) while we're sweeping the same page. The main
thread first creates a filler object that the release-stores the new
map. Therefore it's important to barrier load the map word of live
objects from the sweeper thread.

BUG=none
R=ulan@chromium.org,hpayer@chromium.org
LOG=n

[ulan, 2014-12-11 14:51:34]

lgtm

"The main thread first creates a filler object"
Did you mean "sweeper thread"?

[jochen (gone - plz use gerrit), 2014-12-11 14:57:42]

On 2014/12/11 at 14:51:34, ulan wrote:
> lgtm
> 
> "The main thread first creates a filler object"
> Did you mean "sweeper thread"?

no, the main thread. If we shrink an object, we also create a filler object for
the now unused space.

[jochen (gone - plz use gerrit), 2014-12-11 14:57:47]

The CQ bit was checked by jochen@chromium.org

[commit-bot: I haz the power, 2014-12-11 14:58:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/797623002/1

[commit-bot: I haz the power, 2014-12-11 15:25:30]

Committed patchset #1 (id:1)