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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