Migrate FileSystemCallbacks 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, FileSystemCallbacks wrapper class is migrated to use
simple (BindOnce) callbacks, one for success, another for failure.

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

BUG=933878

Change-Id: I8fae7534be65f992b8b2c5a47cc08a6ab1192502
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529470
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642139}
8 files changed