This returns a server error on setting breakpoints if the
agent is disabled.

Also-by: bmeurer@chromium.org
Fixed: chromium:1202534
Change-Id: I87c80a4bd785fa5c59a8dd0d5ac5f4b31b015ed8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2874662
Commit-Queue: Kim-Anh Tran <kimanh@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Auto-Submit: Kim-Anh Tran <kimanh@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74399}