Switch to headless chrome from QtWebkit driver

We recently had some overlapping elements that made it impossible to click buttons / dropdowns.

Black-box tests should not allow to click on hidden elements. More context - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11639#note_30482800.

We should switch to headless chrome as soon as possible.

Related https://gitlab.com/gitlab-org/gitlab-ce/issues/30876

I'd like to change CE backend engine as well.

This is implemented in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11680!

assigned to @grzesiek

changed milestone to %9.2