Revert "Reland "FilePathWatcher: Synchronously start watching on Mac""

This reverts commit ecb96d59.

Reason for revert: This change is unfortunately causing test flakes again, so need to revert and find the true cause of the flakiness before re-landing again. (crbug.com/332592798)

Original change's description:
> Reland "FilePathWatcher: Synchronously start watching on Mac"
>
> This is a reland of commit b8bf2ad0
>
> The original CL was reverted due to a failing browsertest
> (https://ci.chromium.org/ui/p/chromium/builders/findit/test-single-revision/3436/overview).
>
> It was later determined that this specific test was flaky (https://crbug.com/40939929).
>
> This flake has been resolved as a result of CL: crrev.com/c/5205950
>
> Now that the failing / flaky test has been fixed, this change is ready to re-land in order to unblock FSA Change Observers implementation on Mac.
>
> Original change's description:
> > FilePathWatcher: Synchronously start watching on Mac
> >
> > This CL includes two key changes:
> >
> > 1) Moves all manipulation of the event stream (start, stop, etc)
> >    to the sequence the FilePathWatcher lives on
> >
> > Prior to this CL, the event stream was manipulated on the dispatch
> > queue. This does not appear to have been necessary. Moving the stream
> > manipulation to the sequence the FilePathWatcher lives on allows the
> > event stream to be started synchronously, from the Watch() method
> >
> > 2) Use kFSEventStreamEventIdSinceNow
> >
> > Prior to this CL, the event stream was started via async dispatch in
> > the Watch() method. FSEventsGetCurrentEventId() was used to
> > synchronously get the ID of the most recent (system-wide) event, to
> > ensure that any changes between when Watch() returned and when the
> > stream asynchronously started would be played back as historical
> > events. See the documentation of the sinceWhen field here:
> > https://developer.apple.com/documentation/coreservices/1443980-fseventstreamcreate
> >
> > Since the stream is now started synchronously,
> > kFSEventStreamEventIdSinceNow is sufficient for ensuring that all
> > changes after Watch() returns are noticed
> >
> > Note that creating a stream with kFSEventStreamEventIdSinceNow will
> > no longer fire a "history done" event shortly after stream creation.
> > Any callers which relied on the this behavior may break.
> > See the documentation here:
> > https://developer.apple.com/documentation/coreservices/kfseventstreameventflaghistorydone
> >
> > Bug: 1019297
> > Change-Id: Ib720f8b19d0309714d153de974404fab25b16894
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4814677


> > Reviewed-by: Wez <wez@chromium.org>
> > Commit-Queue: Austin Sullivan <asully@chromium.org>
> > Reviewed-by: Robert Sesek <rsesek@chromium.org>
> > Cr-Commit-Position: refs/heads/main@{#1218981}
>
> Bug: 1019297
> Change-Id: I5d9297c7b3e187bcfdaeaaaf5946984f082b1877
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5274142


> Commit-Queue: Christine Hollingsworth <christinesm@chromium.org>
> Reviewed-by: Daseul Lee <dslee@chromium.org>
> Reviewed-by: Wez <wez@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1281149}

Bug: 1019297
Change-Id: I93b57f4e248fdd79132d6b0e9d92d4aa6b15e1e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5424818


Commit-Queue: Mark Mentovai <mark@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Daseul Lee <dslee@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Auto-Submit: Christine Hollingsworth <christinesm@chromium.org>
Reviewed-by: Christine Hollingsworth <christinesm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283981}