FilePathWatcher: Report Usage Changes

Update the FilePathWatcher interface to support watching usage changes.
It adds:
- current_usage - returns the number of underlying OS resources being
  used. The number is relative to the quota_limit. This CL implements it
  on every platform.
- quota_limit - returns the max amount of resources the FilePathWatcher
  is allowed to use. This is to be determined and currently implemented
  by just returning zero.
- usage_callback - passed to the FilePathWatcher constructor and called
  any time the current_usage changes. Currently unimplemented and will
  be implemented in a follow up CL.

These additions will be used to implement change observer_limits.

Bug: 338457523
Change-Id: Ic71421ff692749563020fdada08039386f15f502
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5968293
Reviewed-by: Daseul Lee <dslee@chromium.org>
Commit-Queue: Nathan Memmott <memmott@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1376661}