Always trigger OnPrefetchCompletedOrFailed

This CL always calls
`PrefetchContainer::Observer::OnPrefetchCompletedOrFailed` when
`PrefetchContainer::OnPrefetchComplete()` is called. (Previously it
wasn't called if `IsDecoy()` is true)

This is to make the following three methods called under the same
condition:

- `PrefetchService::OnPrefetchResponseCompleted()` (when
  `PrefetchContainer` is non-null)
- `PrefetchContainer::OnPrefetchComplete()`
- `PrefetchContainer::Observer::OnPrefetchCompletedOrFailed()`

To keep the behavior when switching the caller of
`PrefetchService::OnPrefetchResponseCompleted()` to
PrefetchContainer::Observer` in
https://chromium-review.googlesource.com/c/chromium/src/+/6336991.

This CL adds a `IsDecoy()` check to keep the existing PrefetchHandle's
behavior, just in case.

Bug: 400761083
Change-Id: I37b2df8248452bf0a99039e1681520b5239cc8b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6703355
Reviewed-by: Taiyo Mizuhashi <[email protected]>
Reviewed-by: Ken Okada <[email protected]>
Commit-Queue: Hiroshige Hayashizaki <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1486614}

• content/browser/preloading/prefetch/prefetch_container.cc[diff]
• content/browser/preloading/prefetch/prefetch_container.h[diff]
• content/browser/preloading/prefetch/prefetch_handle_impl.cc[diff]