[FileSystem]: Rewrite FileWriterDelegate using BlobReader rather than URLRequest.

Not perfect, since this really should be going through the mojo interface
for dealing with blobs, but a lot better than going through blob URLs.

Tbr: reillyg@chromium.org
Bug: 859594, 868462, 804546
Change-Id: Ifd7cd11b7bf4432eff41bdef00edd079d31e49c6
Reviewed-on: https://chromium-review.googlesource.com/1164523
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581790}
32 files changed