Fix incorrect UMA logging for pre-migration fetch keepalive requests.

This CL moves the UMA logging for
`FetchKeepAlive.Requests.{Started,Succeeded,Failed}.Renderer` from
`blink::ResourceFetcher` into `blink::ResourceLoader`, as the response
and error handling after a document is detached does not always happen
in the former component

This CL also adds some browsertest to ensure the pre-migration logging
works as expected.

OBSOLETE_HISTOGRAM[FetchKeepAlive.Requests.Failed.Renderer]=Replaced by FetchKeepAlive.Requests2.Failed.Renderer
OBSOLETE_HISTOGRAM[FetchKeepAlive.Requests.Succeeded.Renderer]=Replaced by FetchKeepAlive.Requests2.Succeeded.Renderer
OBSOLETE_HISTOGRAM[FetchKeepAlive.Requests.Started.Renderer]=Replaced by FetchKeepAlive.Requests2.Started.Renderer
OBSOLETE_HISTOGRAM[FetchKeepAlive.Requests.Total.Renderer]=Replaced by FetchKeepAlive.Requests2.Total.Renderer
OBSOLETE_HISTOGRAM[FetchKeepAlive.Requests.Succeeded.Browser]=Replaced by FetchKeepAlive.Requests2.Succeeded.Browser
OBSOLETE_HISTOGRAM[FetchKeepAlive.Requests.Total.Browser]=Replaced by FetchKeepAlive.Requests2.Total.Browser
OBSOLETE_HISTOGRAM[FetchKeepAlive.Requests.Failed.Browser]=Replaced by FetchKeepAlive.Requests2.Failed.Browser
OBSOLETE_HISTOGRAM[FetchKeepAlive.Requests.Started.Browser]=Replaced by FetchKeepAlive.Requests2.Started.Browser


Bug: 40236167
Change-Id: I08a74d3800d3d0711426345880ce12369773f994
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401537


Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Ming-Ying Chung <mych@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280497}