Only allow background tabs if the underlying input event allows it

If a page takes clickjacks a regular click, don't allow opening
background tabs from anchor elements.

BUG=431335
TEST=none, content_shell doesn't do tabs...
R=mkwst@chromium.org

Review URL: https://codereview.chromium.org/717133002

git-svn-id: svn://svn.chromium.org/blink/trunk@185205 bbb929c8-8fbe-4397-9dbb-9b2b20218538