Fix source SiteInstance computation for SiteInstanceGroups

The NavigationRequest constructor currently has a case where if the
navigation originates from a BeginNavigation IPC, the source
SiteInstance is set to the current RenderFrameHost's
SiteInstance. This is needed to keep navigations to about: or data:
URLs in the initiator's SiteInstance, and works due to the assumption
that even if the initiator frame is different from the frame that
received the BeginNavigation IPC, the two will always be in the same
SiteInstance.

With SiteInstanceGroups, that assumption no longer holds, as the
initiator frame and the navigating frame could be in different
SiteInstances in the same SiteInstanceGroup. So, this CL fixes this
logic to instead look up the source SiteInstance based on the
initiator frame token, using the GetSourceSiteInstanceFromFrameToken()
mechanism that was introduced for a similar case with proxy
navigations in
https://chromium-review.googlesource.com/c/chromium/src/+/5436609. The
idea is that if the initiator RFH is still alive, we get the source
SiteInstance from it, otherwise we get it from
NavigationStateKeepAlive which stores the source SiteInstance if the
initiator had gone away.

This is covered by the GrandchildToAboutBlank_ABB_CrossSite test
running with the default SiteInstanceGroups feature, which will be
introduced in
https://chromium-review.googlesource.com/c/chromium/src/+/6203249.

Note that in an earlier approach, we considered using
SetSourceSiteInstanceToInitiatorIfNeeded() to look up the source
SiteInstance purely based on the initiator origin, but that turns out
to be problematic, because SetSourceSiteInstanceToInitiatorIfNeeded()
relies on looking up an existing SiteInstance via
GetRelatedSiteInstance() based solely on a URL obtained from the
initiator origin. This may end up returning an incorrect SiteInstance
if the initiator uses an effective URL, or if there's a mismatch with
any other SiteInfo bits, which was seen in practice with at least the
PDF and sandboxed bits in existing
tests. SetSourceSiteInstanceToInitiatorIfNeeded() seems to only be
safe for creating a new related SiteInstance when a source
SiteInstance is required (e.g. after session restore), not for looking
up an existing one. There is actually a bug filed already that notes
this at https://crbug.com/349972037.

Bug: 390571607
Change-Id: Id3b155762e7eaefcbfc7c1a40aef4fce0a281482
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6559448
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Sharon Yang <yangsharon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1462847}