Only update icon if the new one is not null.

Only update icon if the new one is not null on x11, no-op for null icon (follow the same way as Windows platform does in HWNDMessageHandler::SetWindowIcons)

BUG=478714
.

Committed: https://crrev.com/eb6d82cd81c4e0a4dc528e98630283b17e17e2d9
Cr-Commit-Position: refs/heads/master@{#329704}

[xiaoling, 2015-05-13 19:18:23]

xiaolingbao@chromium.org changed reviewers:
+ erg@chromium.org

[Elliot Glaysher, 2015-05-13 19:52:56]

lgtm

[xiaoling, 2015-05-13 20:05:19]

The CQ bit was checked by xiaolingbao@chromium.org

[commit-bot: I haz the power, 2015-05-13 20:06:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138343003/20001

[commit-bot: I haz the power, 2015-05-13 20:09:48]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-13 20:10:43]

Patchset 2 (id:??) landed as
https://crrev.com/eb6d82cd81c4e0a4dc528e98630283b17e17e2d9
Cr-Commit-Position: refs/heads/master@{#329704}