Fix a potential null deref in XSSAuditorDelegate.

Fix a potential null deref in XSSAuditorDelegate.

The ASAN bots say this causes a crash; I can't replicate it locally, but
I believe that the combination of `document.write` and synchronous
`javascript:` URL navigation could cause the auditor to trigger while the
document is detaching. This patch adds a small check.

BUG=668772
Committed: https://crrev.com/1d7a2e8d1b446d3ec4b55932d623e1ffe933fd47
Cr-Commit-Position: refs/heads/master@{#434927}

[Mike West, 2016-11-28 21:03:22]

mkwst@chromium.org changed reviewers:
+ tsepez@chromium.org

[Mike West, 2016-11-28 21:03:23]

WDYT, Tom?

[Tom Sepez, 2016-11-28 21:20:21]

Couldn't hurt.  Lets see if it solves it.  LGTM.

[Mike West, 2016-11-29 06:26:33]

The CQ bit was checked by mkwst@chromium.org

[commit-bot: I haz the power, 2016-11-29 06:26:55]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-11-29 07:46:01]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1480400793107840, "parent_rev":
"861dff2ed55152de12cb5a2fa35727392c57211c", "commit_rev":
"6961b2c99cee9160ad68c429b9b2e903a8084ecc"}

[commit-bot: I haz the power, 2016-11-29 07:46:28]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-11-29 07:49:36]

Description was changed from

==========
Fix a potential null deref in XSSAuditorDelegate.

The ASAN bots say this causes a crash; I can't replicate it locally, but
I believe that the combination of `document.write` and synchronous
`javascript:` URL navigation could cause the auditor to trigger while the
document is detaching. This patch adds a small check.

BUG=668772
==========

to

==========
Fix a potential null deref in XSSAuditorDelegate.

The ASAN bots say this causes a crash; I can't replicate it locally, but
I believe that the combination of `document.write` and synchronous
`javascript:` URL navigation could cause the auditor to trigger while the
document is detaching. This patch adds a small check.

BUG=668772

Committed: https://crrev.com/1d7a2e8d1b446d3ec4b55932d623e1ffe933fd47
Cr-Commit-Position: refs/heads/master@{#434927}
==========

[commit-bot: I haz the power, 2016-11-29 07:49:37]

Patchset 1 (id:??) landed as
https://crrev.com/1d7a2e8d1b446d3ec4b55932d623e1ffe933fd47
Cr-Commit-Position: refs/heads/master@{#434927}