Update QA Dockerfile now that Chrome headless no longer in beta

We're currently using the google-chrome-beta_current_amd64.deb beta release image from Google's CDN. Now that Chrome 59 is stable, we can use google-chrome-stable_59.0.3071.86-1_amd64.deb and lock the current version rather than relying on an alias to the current beta as we do here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11680/diffs#6de33ea8c75909f59d02039e8526d7a0d93aa49c_8_13

Related to #30876 (moved), gitlab-qa#48 (closed), and !11680 (merged)

/cc @grzesiek

created branch 33538-update-ci-dockerfile-now-that-chrome-headless-no-longer-in-beta

mentioned in merge request !12071 (merged)

changed title from Update CI Dockerfile now that Chrome headless no longer in beta to Update QA Dockerfile now that Chrome headless no longer in beta

changed milestone to %9.4

closed via merge request !12071 (merged)

mentioned in commit 6ff571aa