Don't UnshiftGrey any black objects

Don't UnshiftGrey any black objects

R=hpayer@chromium.org
BUG=

Committed: https://crrev.com/dd5a93cd5bcfc209f138db7d9df9306da539afdf
Cr-Commit-Position: refs/heads/master@{#28582}

[Erik Corry, 2015-05-13 20:03:29]

The CQ bit was checked by erik.corry@gmail.com to run a CQ dry run

[commit-bot: I haz the power, 2015-05-13 20:03:40]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1128683006/1

[commit-bot: I haz the power, 2015-05-13 20:03:41]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-05-13 20:03:42]

Dry run: No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Erik Corry Chromium.org, 2015-05-13 20:06:46]

The CQ bit was checked by erikcorry@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-05-13 20:07:03]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1128683006/1

[commit-bot: I haz the power, 2015-05-13 21:05:43]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-05-13 21:05:44]

Dry run: This issue passed the CQ dry run.

[Hannes Payer (out of office), 2015-05-15 15:26:06]

https://codereview.chromium.org/1128683006/diff/1/src/heap/incremental-markin...
File src/heap/incremental-marking.cc (right):

https://codereview.chromium.org/1128683006/diff/1/src/heap/incremental-markin...
src/heap/incremental-marking.cc:190: if (start_offset < object_size) {
As discussed offline, let's figure out the reason first why the invariant *here
should be just grey object* does not hold.

[Erik Corry Chromium.org, 2015-05-19 13:50:51]

3 days later nobody has time to figure this out.  The proposed change looks like
a safe default to me: The grey bits will cause the array to be rescanned.  It
doesn't stop us later working out what is going on.

This could be related to the revert of the new write barrier. That could be
causing more marking deque overflows, which makes this more likely to happen.

[Hannes Payer (out of office), 2015-05-19 15:03:52]

On 2015/05/19 13:50:51, Erik Corry Chromium.org wrote:
> 3 days later nobody has time to figure this out.  The proposed change looks
like
> a safe default to me: The grey bits will cause the array to be rescanned.  It
> doesn't stop us later working out what is going on.
> 
> This could be related to the revert of the new write barrier. That could be
> causing more marking deque overflows, which makes this more likely to happen.

Lgtm, can you please investigate why the invariant is not true.

[Erik Corry Chromium.org, 2015-05-22 10:23:52]

The CQ bit was checked by erikcorry@chromium.org

[commit-bot: I haz the power, 2015-05-22 10:24:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1128683006/1

[commit-bot: I haz the power, 2015-05-22 11:07:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-22 11:07:53]

Patchset 1 (id:??) landed as
https://crrev.com/dd5a93cd5bcfc209f138db7d9df9306da539afdf
Cr-Commit-Position: refs/heads/master@{#28582}