ServiceWorker: Add test for subsequent registrations from a different iframe

This is a follow-up patch for https://codereview.chromium.org/517493002/
(see (b) in that CL's description and comment#5 in the BUG)


BUG=406240
TEST=run_webkit_tests.py --debug http/tests/serviceworker/multiple-register.html

Review URL: https://codereview.chromium.org/512273003

git-svn-id: svn://svn.chromium.org/blink/trunk@181283 bbb929c8-8fbe-4397-9dbb-9b2b20218538