service worker: Refactor blob reading out of ServiceWorkerURLRequestJob

ServiceWorkerURLRequestJob has two distinct states when there is a response
from the service worker: it can be reading a blob or a stream.  To clarify the
states, factor out blob and stream response reading into dedicated classes.

This should also help netlog fit better with the superclass's "request
start" vs "reading response" phases.

Review-Url: https://codereview.chromium.org/2290453005
Cr-Commit-Position: refs/heads/master@{#415293}
5 files changed