Simplify ServiceWorkerContainer's access to its window.

Simplify ServiceWorkerContainer's access to its window.

Actually two refactorings for navigator.serviceWorker.ready which is
sensitive to lifetimes. No change in behavior.

First, ServiceWorkerContainer is a DOMWindowLifecycleObserver to
access a window for postMessage. But it already has an execution
context that can provide the DOMWindow. Use that instead.

Second, adds an unrelated assertion that the container's
ServiceWorkerContainerClient has been cleaned up by the time that the
container is deleted. To do otherwise is to leave the
ServiceWorkerContainerClient with a dangling pointer to the container.

BUG=363967
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175973

[dominicc (has gone to gerrit), 2014-06-10 22:00:05]

PTAL

Basically clean-up ahead of making .ready work for the waiting Service Worker.

[michaeln, 2014-06-10 23:46:50]

lgtm

[dominicc (has gone to gerrit), 2014-06-11 18:06:31]

The CQ bit was checked by dominicc@chromium.org

[commit-bot: I haz the power, 2014-06-11 18:07:02]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/dominicc@chromium.org/328663010/20001

[commit-bot: I haz the power, 2014-06-11 19:17:40]

Change committed as 175973