arc: Add metrics for removable media unmount in ARCVM

This CL adds some metrics to know if the host-and-guest synchronization
in ArcVolumeMounterBridge::PrepareForRemovableMediaUnmount() (added in
CL:5845688) is working okay.

BUG=b:317944073
TEST=Manually check chrome://histograms.

Change-Id: I3830f6a6111a3da80e58a58307c13f169a0bf3ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5922496
Reviewed-by: François Degros <fdegros@chromium.org>
Auto-Submit: Momoko Hattori <momohatt@chromium.org>
Reviewed-by: Satoshi Niwa <niwa@chromium.org>
Reviewed-by: Muhammad Hasan Khan <mhasank@chromium.org>
Commit-Queue: Muhammad Hasan Khan <mhasank@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1367645}