Refactor NavigationRequest selection in RenderFrameHost

This is a preparatory CL for enforcing having a valid NavigationRequest at
commit time. It makes clearer when NavigationRequests are deemed non-valid, and
when a new NavigationRequest must be created at commit time.

Change-Id: I02bcc43fb46378d4507ba90beaf0b007b909b048
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1493693
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639912}
6 files changed