Download: Use base::CheckedObserver in a few classes.

Use CheckedObserver to replace the unchecked ones, so we can get better
stack trace of a use-after-free crash in DownloadDriverImpl.

Bug: 1303152
Change-Id: I2c23967405033f5b4947001ac3ce5075be2584a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3508189


Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#978763}