Enable Service Workers, including the onfetch event, by default.

BUG=364627
TEST=virtual/stable/webexposed

Review URL: https://codereview.chromium.org/680473002

git-svn-id: svn://svn.chromium.org/blink/trunk@184340 bbb929c8-8fbe-4397-9dbb-9b2b20218538