Reactivate building browser_tests.exe for Windows official builds.

Now that the fix to the build-warning seen earlier,
https://codereview.chromium.org/333443002 has landed, reactivate building
browser_tests.exe for official Windows builds.

BUG=379986
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=276835

[anandc, 2014-06-12 17:43:44]

Hi Michael, PTAL.

I've also kicked off an official Chrome build on my Windows desktop, to see if
this will succeed and that there are no more warnings like we saw the last time
(https://codereview.chromium.org/333443002). 
That may take many hours to complete and could fail for other reasons, so not
sure if we want to block on the results of that. If it does succeed, that would
be a good sign.

Thanks.

[Michael Moss, 2014-06-12 18:06:01]

lgtm

[anandc, 2014-06-12 18:17:52]

The CQ bit was checked by anandc@chromium.org

[commit-bot: I haz the power, 2014-06-12 18:21:13]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/anandc@chromium.org/328423002/1

[commit-bot: I haz the power, 2014-06-12 22:45:55]

Change committed as 276835