Migrate FileWriterCallbacks to use simple success/failure callbacks

This CL is a continuation of the effort to simplify the callbacks
implementation in t_p/b/renderer/modules/filesystem/file_system_callbacks.h
At this time, FileWriterCallbacks wrapper class is migrated to use
simple (BindOnce) callbacks, one for success, another for failure.

Note that FileWriterCallbacks did not share the same callback completion
implementations as some other methods that use the templates in
async_callback_helper.h, and its success completion callback body
was inlined in file_entry.cc.

R=mek@chromium.org
CC=​blink-reviews-vendor@chromium.org

BUG=933878

Change-Id: I381d3ebe9c45e124740be423c20127c884b60a33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529481
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#642658}
6 files changed