Add a new interface to get URLLoaderFactory for components/download

For download service, we are going to pass a class that retrieves URLLoaderFactory.
This CL defines a common interface for that purpose.
And the implementation can support either a blob url or a network url.

BUG=803135

Change-Id: I505e025fc54553753180f05b97fbd3c4b654253a
Reviewed-on: https://chromium-review.googlesource.com/1007974
Commit-Queue: Min Qin <[email protected]>
Reviewed-by: Bo <[email protected]>
Reviewed-by: Xing Liu <[email protected]>
Cr-Commit-Position: refs/heads/master@{#550192}
12 files changed