[Contextual Tasks] Add finch param for host replacement

This patch adds a "forced-embedded-page-host" param to the
kContextualTasks experiment flag which allows us to override the host
that would otherwise be loaded. The path and params for the URL
otherwise remain unchanged. If the specified host is not a google.com
host, the value is unset.

Bug: 468400006
Change-Id: Idc79fba72ca2e0a631673c9fc5faa769645959d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7256441
Reviewed-by: Duncan Mercer <mercerd@google.com>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1558356}