Return null when window.open blocks a popup.

Return null when window.open blocks a popup.

window.open [1] steps 4 and 9.1 say to return null, not undefined,
when window.open decides to not open a window. This brings Chrome into
line with Edge and Firefox.

[1] https://html.spec.whatwg.org/#dom-open

BUG=652640

[dominicc (has gone to gerrit), 2016-10-06 07:32:24]

The CQ bit was checked by dominicc@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-06 07:32:42]

Dry run: 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, 2016-10-06 08:00:24]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-06 08:00:25]

Dry run: Try jobs failed on following builders:
  cast_shell_linux on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/cast_shell_linu...)