BulkPinningDialog: Display current free space

BUG=b:284047242

Change-Id: Iccdb6a81113545a46f2a1db6724027766b1c482d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4560288


Commit-Queue: Ben Reich <benreich@chromium.org>
Auto-Submit: François Degros <fdegros@chromium.org>
Reviewed-by: Ben Reich <benreich@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148339}