Garden-o-matic: Stop the tab explosion!

This patch stops garden-o-matic from opening links in new tabs. Users
that like this feature are free to use the middle-click or ctrl+click
to open links in a new tab. This will increase gardener happiness by
34% (sample size of 1, measured today).

R=ojan, dglazkov

Review URL: https://codereview.chromium.org/135303006

git-svn-id: svn://svn.chromium.org/blink/trunk@165341 bbb929c8-8fbe-4397-9dbb-9b2b20218538