FspCache: Remove files upon eviction if there are no open requests

Previously, upon eviction, files were marked as evicted but not removed.
They would have been removed if RemoveEvictedItems() was called but
since there's no point keeping files around, let's remove them at
eviction if there are no open LocalFDs. That is, remove the file if
there are no requests in the middle of reading/writing to the file.

If there are open LocalFDs, then the file will be removed once those
read/write requests have completed and the final LocalFD is closed.

Note: new read/write requests will be denied access to a LocalFD as
introduced in CL:5546857 so won't affect when the file gets removed.

Add a ContentCache::Observer so the tests can observe when a file has
been removed.

Test: unit_tests --gtest_filter=FileSystemProviderContentCacheImplTest
Bug: b/339322536
Change-Id: I4ca4b354e05d1386606462235751187b44502f59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5547619
Reviewed-by: Ben Reich <benreich@chromium.org>
Commit-Queue: Cassy Chun-Crogan <cassycc@google.com>
Cr-Commit-Position: refs/heads/main@{#1303602}