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=

Review URL: https://codereview.chromium.org/721793002

git-svn-id: svn://svn.chromium.org/blink/trunk@185220 bbb929c8-8fbe-4397-9dbb-9b2b20218538