Fix crash during EmbeddedWorkerInstance startup sequence failures

Once EWInstance startup calls the callback, it's possible that the
underlying ServiceWorkerVersion is destroyed, hence destroying
|this|. We must guard against that.

Also some failure points in the startup sequence weren't calling
OnStopped() as expected.

BUG=529520, 531345

Review URL: https://codereview.chromium.org/1327723005

Cr-Commit-Position: refs/heads/master@{#349368}