Check whether popups are allowed before creating a new tab via loadURLExternally

There was already a test for when with ctrl-click on a link with a target parameter. If there's none, a different code path is taken. This patch & test covers this different case.

BUG=259755
R=japhet@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21515002

git-svn-id: svn://svn.chromium.org/blink/trunk@155333 bbb929c8-8fbe-4397-9dbb-9b2b20218538