task posting v3: remove includes of runner handles and IWYU task runners

Turns out there's a lot of includes, so these will have to be removed
before deleting the implementation of the task runner handles.

To allow the deletion of the task runner handle headers, add
the sequenced/thread task runner handles where they are used in
the codebase with scripts.

This was done with an automated change, with a few touchups afterwards.
The code for the mass-refactor changes are here:
python:
https://paste.googleplex.com/5534570878337024
shell:
https://paste.googleplex.com/6466750748033024

In terms of touchups:
- add sequenced/thread task runner handles to
  the third_party/blink/public/DEPS, because multiple files were using
  it transitively anyways.
- rewrite certain parts of the codebase which used
  ThreadTaskRunnerHandles instead of CurrentDefaultHandles.
- fix a compile issue with forward-declaration in
  extensions/browser/extension_file_task_runner.h.

AX-Relnotes: n/a.
Bug: 1026641
Change-Id: I737ef32aee4e77c21eaa3a2bdc403a28322cf1b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4133323
Owners-Override: Gabriel Charette <gab@chromium.org>
Commit-Queue: Sean Maher <spvw@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1090532}
NOKEYCHECK=True
GitOrigin-RevId: e672a665ffa8fe4901184f03922e2cc548399da5
5 files changed