[prerender] Create prerender SiteInstance from the prerender URL.

This CL ensures that a prerender's initial SiteInstance is created with
a SiteInfo derived from the prerendering URL.

Currently, a PrerenderHost initializes its frame tree with a
SiteInstance that has an empty SiteInfo (created via
SiteInstanceImpl::Create()). This forces process selection to always
choose a new renderer for the prerender.

This behavior is incompatible with DSE-related performance features like
ProcessPerSite (which co-hosts DSE pages in one process) and DSE
keep-alive (which keeps an empty DSE renderer alive for reuse). A
prerender for a DSE warmup page was unable to reuse either of these
existing, warm processes, defeating their purpose.

This patch fixes the issue by using
SiteInstanceImpl::CreateForURL(prerendering_url) instead. This provides
the SiteInstance with the correct SiteInfo, allowing standard process
selection logic to run. The prerender can now correctly reuse an
existing DSE renderer if one is available via keep-alive or
ProcessPerSite.

Bug: 444530329
Change-Id: Iba037557d435e321c87fe6db5cfff1177da1cb11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6943575
Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Jiacheng Guo <gjc@google.com>
Cr-Commit-Position: refs/heads/main@{#1517699}