Check document.firstChild() exists to avoid nullptr deref

Check document.firstChild() exists to avoid nullptr deref

document.firstChild() can return nullptr when document has no Children.
This CL is in response for the comment in
https://codereview.chromium.org/1910433002/

BUG=405707
Committed: https://crrev.com/04c7245bc4ff706aaf9c1af480fd3065faf7478b
Cr-Commit-Position: refs/heads/master@{#388984}

[kochi, 2016-04-21 09:58:53]

Description was changed from

==========
Check document.firstChild() exists to avoid nullptr deref

document.firstChild() can return nullptr when document has no Children.


BUG=405707
==========

to

==========
Check document.firstChild() exists to avoid nullptr deref

document.firstChild() can return nullptr when document has no Children.


BUG=405707
==========

[kochi, 2016-04-21 09:58:53]

kochi@chromium.org changed reviewers:
+ esprehn@chromium.org

[kochi, 2016-04-21 10:02:14]

Elliott, I tried to write a unit test for this and found
that this code literally seems dereferencing null pointer,
but actually the ABI passes nullptr as Node& reference,
and NodeTraversal::startsAt(Node*) can handle the nullptr
properly, so this didn't crash.  The code before my change
also did NodeTraversal::startsAt(document().firstChild()).

Anyway, it's confusing and it is better to have nullptr
check here.

[kochi, 2016-04-21 10:20:16]

Description was changed from

==========
Check document.firstChild() exists to avoid nullptr deref

document.firstChild() can return nullptr when document has no Children.


BUG=405707
==========

to

==========
Check document.firstChild() exists to avoid nullptr deref

document.firstChild() can return nullptr when document has no Children.
This CL is in response for the comment in
https://codereview.chromium.org/1910433002/

BUG=405707
==========

[esprehn, 2016-04-21 18:08:40]

lgtm

[kochi, 2016-04-21 23:02:33]

The CQ bit was checked by kochi@chromium.org

[commit-bot: I haz the power, 2016-04-21 23:03:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1908893002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1908893002/1

[commit-bot: I haz the power, 2016-04-22 00:54:01]

Description was changed from

==========
Check document.firstChild() exists to avoid nullptr deref

document.firstChild() can return nullptr when document has no Children.
This CL is in response for the comment in
https://codereview.chromium.org/1910433002/

BUG=405707
==========

to

==========
Check document.firstChild() exists to avoid nullptr deref

document.firstChild() can return nullptr when document has no Children.
This CL is in response for the comment in
https://codereview.chromium.org/1910433002/

BUG=405707
==========

[commit-bot: I haz the power, 2016-04-22 00:54:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-04-22 19:42:46]

Description was changed from

==========
Check document.firstChild() exists to avoid nullptr deref

document.firstChild() can return nullptr when document has no Children.
This CL is in response for the comment in
https://codereview.chromium.org/1910433002/

BUG=405707
==========

to

==========
Check document.firstChild() exists to avoid nullptr deref

document.firstChild() can return nullptr when document has no Children.
This CL is in response for the comment in
https://codereview.chromium.org/1910433002/

BUG=405707

Committed: https://crrev.com/04c7245bc4ff706aaf9c1af480fd3065faf7478b
Cr-Commit-Position: refs/heads/master@{#388984}
==========

[commit-bot: I haz the power, 2016-04-22 19:42:48]

Patchset 1 (id:??) landed as
https://crrev.com/04c7245bc4ff706aaf9c1af480fd3065faf7478b
Cr-Commit-Position: refs/heads/master@{#388984}