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

Note that VoidCallbacks did not share the same callback completion
implementation as some other methods that use the templates in
async_callback_helper.h. For this reason, two non-template methods
were added to async_callback_helper.h, to be used exclusively by
VoidCallbacks (no hard enforcement, only the documentation in the
file header).

This is the last CL before we are able to clean up sync_callback_helper.h
a lot, as well as file_system_callbacks.h.

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

BUG=933878

Change-Id: I16c1d2e1defc9afeff0a616fe0b2fcc00a2d5c65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529482
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#642772}
11 files changed