Fix usages of ScopedBlockingCall

Create named objects in chaps_slot_session.cc when using
ScopedBlockingCall to persist the effect of the object
to the entire scope.

Add [[nodiscard]] to ScopedBlockingCall and
ScopedBlockingCallWithBaseSyncPrimitives classes to prevent
this kind of bugs in the future.

Fixed: b/261589658
Test: Compilation
Change-Id: I0de5ea20c15802e644a544ee073d5587c48fc47e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4208750
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Kyle Charbonneau <kylechar@chromium.org>
Commit-Queue: Michael Ershov <miersh@google.com>
Cr-Commit-Position: refs/heads/main@{#1099724}