Adds null check for isHTMLTableCellElement for AX presentational Role.

Adds null check for isHTMLTableCellElement for AX presentational Role.

Crash was reported due to the CL, 1039873002, and it was caused by
missing null check and sending wrong type. This patch adds null check
and change the parameter type.
The related CL:crrev.com/1039873002

BUG=474409
R=dmazzoni

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=193248

[je_julie(Not used), 2015-04-07 04:04:53]

je_julie.kim@samsung.com changed reviewers:
+ dmazzoni@chromium.org

[je_julie(Not used), 2015-04-07 04:04:53]

Hi Dominic,

The crash happened because null check was missed.
Because isHTMLTableCellElement is not auto generated helper element function,
there is no API with pointer param.
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...

I'm sorry to cause crash issue.
For make sure, I've checked all html element helper APIs I used for
presentational Role.

This patch adds null check and update parameter with correct type.
PTAL.

[dmazzoni, 2015-04-07 05:36:55]

The CQ bit was checked by dmazzoni@chromium.org

[dmazzoni, 2015-04-07 05:36:55]

lgtm

[commit-bot: I haz the power, 2015-04-07 05:37:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1052123003/1

[commit-bot: I haz the power, 2015-04-07 07:00:11]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=193248