[suggest-internals] Handle deleted loaders when hardcoding a response

crrev.com/c/5912624 added a mechanism to artificially delay mock
responses. However if the delay is set to a larger value than the
autocomplete stop timer, which stops the provider and deletes the
loader, running the completion callback can cause a crash.

This change exposes a base::WeakPtr in SimpleURLLoader to detect
such scenarios.

Bug: 40254435
Change-Id: Ic83a0a460af12f2e61142b7256b71c4c89be05f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6000840
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Nihar Majmudar <niharm@google.com>
Cr-Commit-Position: refs/heads/main@{#1380359}