Revert of Revert "Only allow background tabs if the underlying input event allows it"

Revert of Revert "Only allow background tabs if the underlying input event allows it" (patchset #1 id:1 of https://codereview.chromium.org/721793002/)

Reason for revert:
Removed failing browser test

Original issue's description:
> Revert "Only allow background tabs if the underlying input event allows it"
> 
> This reverts commit c67fa612fedd3cf79f999ad61f2022dec85ff032, aka r185205.
> 
> Speculative revert to green up browser_tests:
> 
> http://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=browser_tests&tests=PrerenderBrowserTest.PrerenderClickNewBackgroundTab
> 
> TBR=jochen@chromium.org
> BUG=
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=185220

TBR=senorblanco@chromium.org
BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=185318

[jochen (gone - plz use gerrit), 2014-11-13 19:40:53]

Created Revert of Revert "Only allow background tabs if the underlying input
event allows it"

[jochen (gone - plz use gerrit), 2014-11-13 19:41:24]

The CQ bit was checked by jochen@chromium.org

[commit-bot: I haz the power, 2014-11-13 19:41:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/720283002/1

[commit-bot: I haz the power, 2014-11-13 20:47:36]

Committed patchset #1 (id:1) as 185318