Make block on sentinel file temporary.

If the sentinel file reaches 5 bytes, GCPW gets deactivated
permanently. This change enables a timestamp so GCPW gets
deactivated only temporarily. After specified time,
the sentinel file gets deleted and everything goes back to
normal.

Change-Id: I85704c47af044e3d54b2404b256ac6ea76a2b162
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6605402
Reviewed-by: Tien Mai <tienmai@chromium.org>
Reviewed-by: Rakesh Soma <rakeshsoma@google.com>
Commit-Queue: Laura Lucas Alday (xWF) <lalday@google.com>
Cr-Commit-Position: refs/heads/main@{#1470164}