The first PdfHandler job

See also commits in PdfHandler[1], mediawiki/selenium[2] and zuul-config[3].

Created job:

browsertests-PdfHandler-test2.wikipedia.org-linux-firefox-sauce[4]

1: https://gerrit.wikimedia.org/r/#/c/143175/
2: https://gerrit.wikimedia.org/r/#/c/144984/
3: https://gerrit.wikimedia.org/r/#/c/152788/
4: https://integration.wikimedia.org/ci/view/BrowserTests/job/browsertests-PdfHandler-test2.wikipedia.org-linux-firefox-sauce/

Change-Id: Iba027897d2f4ffaadc74efea41188db9d2815f53