[ServiceWorker] Create a new Blob when body.asBody() is called if the size of body is not set.

This patch fixes the wrong size of blob which can be get by calling FetchEvent.request.body.asBody() of the fetch event which body includes a file item.

This change depends on https://codereview.chromium.org/517543002.


BUG=402387

Review URL: https://codereview.chromium.org/514113003

git-svn-id: svn://svn.chromium.org/blink/trunk@181077 bbb929c8-8fbe-4397-9dbb-9b2b20218538