Fix missing metrics for Service Worker Synthetic Responses

This change fixes a bug where page load metrics (e.g., FCP, LCP) were
not being recorded for pages served via Service Worker Synthetic
Responses.

The metric recording in `ServiceWorkerPageLoadMetricsObserver` often
depended on the page being fully service worker controlled
(`kLoadingBehaviorServiceWorkerControlled`). However, synthetic
responses can be served even without a fetch handler, meaning the page
isn't "controlled" in the traditional sense, causing metrics to be
missed.

This patch addresses this by:

1.  **Ensuring Correct Flagging:** Modifying `DocumentLoader` to always
    set the `kLoadingBehaviorServiceWorkerSyntheticResponse` flag when a
    synthetic response is used, independent of the controller mode.
2.  **Updating Observer Logic:** Introducing helper functions in
    `ServiceWorkerPageLoadMetricsObserver` to check for *either*
    service worker control or the presence of the synthetic response
    flag (`IsServiceWorkerControlledOrSyntheticResponseEnabled`). This
    ensures metric recording is triggered correctly for synthetic
    response cases.

The unit test mistakenly set `kLoadingBehaviorServiceWorkerControlled`
in the last CL and that introduced this bug. So added CHECK to the
`IsServiceWorkerSyntheticResponseEnabled()` in ServiceWorkerPLMO to
ensure the correct expectation. Ideally, it's better to have a browser
test for the synthetic response page load metrics, but it requires
another complicated test set up. Let me leave it in a future CL.

Bug: 40240298, 352578800
Change-Id: I624fcc4b9f6e4d7216d4dfa2dcc57c628094b48c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6887127
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Shunya Shishido <sisidovski@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1506918}