Set request_start to response_head in synthetic response

When the navigation is eligible for synthetic response, but the response
header is not stored yet, ServiceWorkerMainResourceLoader just passes
the response head and body to `url_loader_client_`.

In this case, `response_head_` is not involved and we need to set
`request_start` and `request_start_time` manually.

This will fix following TypedNavigationUpgradeThrottleBrowserTest:
- TypedNavigationUpgradeThrottleBrowserTest.SearchQuery_ShouldNotUpgrade
- SearchQuery_TwoWords_ShouldNotUpgrade

Bug: 352578800
Change-Id: Id5235b1b82ccf1fee8c54ac185d792c92ddea314
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6893446
Reviewed-by: Keita Suzuki <suzukikeita@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Shunya Shishido <sisidovski@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1508950}