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=

Review URL: https://codereview.chromium.org/720283002

git-svn-id: svn://svn.chromium.org/blink/trunk@185318 bbb929c8-8fbe-4397-9dbb-9b2b20218538