Revert "Remove the usage of BrowserThread::FILE in the spellchecker files"

This reverts commit 9e1aabd5c25c1d8b76752372167dae7ce8aa0d89.

Reason for revert: SpellcheckServiceBrowserTest.DeleteCorruptedBDICT is
flaky again.

Original change's description:
> Remove the usage of BrowserThread::FILE in the spellchecker files
> 
> The usage of BrowserThread::FILE is deprecated in favor of the
> post_task.h API.
> 
> This is a new attempt at
> https://chromium-review.googlesource.com/c/510018,
> that got reverted because of flaky tests.
> 
> I changed the code to make sure the dictionary is deleted on the
> same task runner is it used in.
> 
> Bug: 689520
> Change-Id: Ie8bfe46de608a4b1944b259007314405c9b56d50
> Reviewed-on: https://chromium-review.googlesource.com/619607
> Commit-Queue: Patrick Monette <pmonette@chromium.org>
> Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#496394}

TBR=rouslan@chromium.org,pmonette@chromium.org
NOTRY=true
NOPRESUBMIT=true

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 689520
Change-Id: I8887b9038ead9a9bc1a3b5102c1a6eadfc5bb36d
Reviewed-on: https://chromium-review.googlesource.com/629776
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Reviewed-by: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496802}
5 files changed