Make as much of NavigationRequest const as possible.

- make some things constructor parameters previous were poked in after
  construction
- move nav_entry_id_ so that all the entry-dependent initializers
  were together
- drive-by fix 1 typo
- move things around a little for initiator_routing_id_
- use make_unique instead of new in 1 spot

I'm pretty sure this is a no-op although there may be ordering issues
that I didn't spot.

Change-Id: Ice1e0d53f2351795da109047d4d16fd1776d3db3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2230181
Commit-Queue: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Auto-Submit: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775790}