ServiceWorker: register() should return the existing registration object if available

Before this patch, register() always returns new registration object
even if the same registration already exists. This patch makes
register() return the existing object if it's available.

Chromium-side change: https://codereview.chromium.org/470843002/

NOTE: ServiceWorkerRegistration is still disabled and this change does
not affect the existing code/behavior.


BUG=396400
TEST=https://codereview.chromium.org/466723002/
NOTRY=true

Review URL: https://codereview.chromium.org/473743002

git-svn-id: svn://svn.chromium.org/blink/trunk@180251 bbb929c8-8fbe-4397-9dbb-9b2b20218538