disk_cache: Make Backend::GetEntryCount asynchronous

This change converts `disk_cache::Backend::GetEntryCount()` to an
asynchronous method. The primary motivation is the planned `SqlBackend`
implementation, which will not maintain the entry count on the backend's
live sequence. Retrieving this value requires a hop to a different
sequence, making a synchronous API unsuitable.

Outside of test code, `GetEntryCount()` is currently only used by the
`BlockFile` backend implementation. Other backends only use this method
in tests. This limited production scope makes the migration to an
asynchronous API manageable.

The change propagates up the call stack to consumers of this API, such
as `GpuDiskCache` and `PnaclTranslationCache`, making their equivalent
`Size()` methods asynchronous as well.

Existing backend implementations now conform to the new asynchronous
interface. For `blockfile::BackendImpl`, the previous synchronous logic
has been moved to a new private method, `GetEntryCountSync()`. A new
`net::TestInt32CompletionCallback` is also introduced to simplify
updating the numerous affected unit tests.

Bug: 422065015
Change-Id: Ic2d8c1e1c3a7a6b86479b33e2e545c138abc77b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6646584
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1474730}