Replace GetWeakPtr() with GetWatcherEntry() in FilePathWatcherImpl

This CL refactors FilePathWatcherImpl to not expose its WeakPtr
directly and instead expose a more high level GetWatcherEntry().
This also enables FilePathWatcherImpl to not expose
GetTaskRunner().

R=wez@chromium.org

Bug: 1366168
Change-Id: I671af5e72965a62a2c09a17d3a0bc394528241c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3934006
Reviewed-by: Wez <wez@chromium.org>
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1060425}