Invoke FileSystemCallbacks's success/error closures directly

Previously, the success and error callbacks in the various classes
in [1] were either invoked immediately or scheduled to run depending on
the execution context's task runner.

Lately, according to offline conversations with mek@chromium.org, it is
possible to get rid of the scheduling logic altogether because an earlier,
change made it so that things like FilesystmeDispatcher::DidReadMetadata
already run on a per-execution context task runner, and presumably they
would already be scheduled properly.

This CL implements that idea, as part of the effort to clean
FileSystemCallbacks's implementation.
It also greatly simplifies FileSystemCallbacksBase, which can be
eliminated in a follow up CL with minimal effort.

[1] //third_party/blink/renderer/modules/filesystem/file_system_callbacks.h

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

BUG=933878

Change-Id: Ib446249bab8ac1c9d9f2e4d11340d258cbac1751
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525101
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641097}
2 files changed