[ServiceWorker] Add LayoutTests for HTMLImports with ServiceWorker

This change depends on https://codereview.chromium.org/687493002/

BUG=427136,418509

Review URL: https://codereview.chromium.org/690573002

git-svn-id: svn://svn.chromium.org/blink/trunk@184633 bbb929c8-8fbe-4397-9dbb-9b2b20218538