Fix Cocoa accessibility callbacks type mismatch

Fix Cocoa accessibility callbacks type mismatch

If you compile without preprocessing (first clang -E then clang, like
ccache does by default without CCACHE_CPP2), you will see warnings like:

you will see warnings like:

../../content/browser/accessibility/browser_accessibility_cocoa.mm:1155:12:error: implicit conversion of NULL constant to 'NSUInteger' (aka
'unsigned long') [-Werror,-Wnull-conversion]
    return __null;
    ~~~~~~ ^~~~~~
           0

It's because we return nil when 0 or NO should be returned instead,
normal clang compile won't catch that for some reason, perhaps because
its checking is less strict for preprocessed code.

Committed: https://crrev.com/8cd33c00d45afc7879d352d471f857c9faab59db
Cr-Commit-Position: refs/heads/master@{#294848}

[Jiang Jiang, 2014-09-15 13:53:35]

jiangj@opera.com changed reviewers:
+ avi@chromium.org

[Jiang Jiang, 2014-09-15 13:53:36]

Avi and Nico: Any of you know why regular clang compile won't catch this? I can
only see warnings when clang is compiling code output by clang -E.

[Jiang Jiang, 2014-09-15 13:55:41]

On 2014/09/15 13:53:36, Jiang Jiang wrote:
> Avi and Nico: Any of you know why regular clang compile won't catch this? I
can
> only see warnings when clang is compiling code output by clang -E.

There are many similar issues, mostly in third_party code, I'm fixing them one
by one so that we at Opera can build with ccache without CCACHE_CPP2 (run second
cpp pass), which will shorten the best case full rebuild time (all cache hit) by
30%.

[Avi (use Gerrit), 2014-09-15 15:47:04]

Looks like someone was way too aggressive with the copy/paste.

LGTM

[Jiang Jiang, 2014-09-15 15:50:03]

The CQ bit was checked by jiangj@opera.com

[commit-bot: I haz the power, 2014-09-15 15:50:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/565383003/1

[commit-bot: I haz the power, 2014-09-15 17:51:33]

Committed patchset #1 (id:1) as 5a00cb7069d93529a4702e48ddae3b0c295223f5

[commit-bot: I haz the power, 2014-09-15 18:03:51]

Patchset 1 (id:??) landed as
https://crrev.com/8cd33c00d45afc7879d352d471f857c9faab59db
Cr-Commit-Position: refs/heads/master@{#294848}