FSO: Omit changedHandle for "disappear", "error", and "unknown" events
This updates the changedHandle field in FileSystemChangeRecord to be
an optional. It also updates the changedHandle() getter to return a
nullptr for disappeared, errored, and unknown change types.
Bug: 377903461
Change-Id: Ie6abe56d9032dbe42aba6f893837e6a43a027980
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6012533
Reviewed-by: Nathan Memmott memmott@chromium.org
Commit-Queue: Rahul Singh rahsin@microsoft.com
Cr-Commit-Position: refs/heads/main@{#1383791}