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This patch removes the requirement for the caller of a BackgroundSync
manager method to be either a service worker or a controlled client.
Now, uncontrolled clients are also able to access the manager, and Blink
informs the browser whether a registration request came from a Service
Worker or not. (Service Workers are still required to have at least one
active window client in order to register sync events; this is enforced
in the browser).

This is part 1 of a three-part blink-chromium-blink dance.
Part 1: https://codereview.chromium.org/1309393003 (this patch)
Part 2: https://codereview.chromium.org/1316743002
Part 3: https://codereview.chromium.org/1314453003

BUG=518884

Review URL: https://codereview.chromium.org/1309393003

git-svn-id: svn://svn.chromium.org/blink/trunk@201230 bbb929c8-8fbe-4397-9dbb-9b2b20218538