Move async_file_system_callbacks.h to t_p/blink/renderer/modules/filesystem

... from t_p/blink/renderer/platform/.

This is a first step to clean up how callbacks are handling in
modules/filesystem.

BUG=933878

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

Change-Id: I0d75062132af40e7e8038e1e90163d3b88d84caa
Reviewed-on: https://chromium-review.googlesource.com/c/1494681
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#636550}
7 files changed