[Permissions] Fix crash

We currently use a separate task runner for the callback that has a
weak_ptr reference to an object living on the original thread. We need
to call this callback - which is a member of the mentioned object - from
the same thread as the model call is originating, so we - for now - use
GetCurrentDefault for the task runner.

Bug: 431313594
Change-Id: Iffb6bc77e44e1f7e5f5ebbc24fbad0bf03761ac7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6732985
Reviewed-by: Elias Klim <elklm@chromium.org>
Commit-Queue: Judith Hemp <hempjudith@google.com>
Cr-Commit-Position: refs/heads/main@{#1486278}