Have NavigationSimulator support browser-initiated navigations

This CL adds support for simulating browser-initiated navigations using
NavigationSimulator. It removes the one usage outside of content/ of
RenderFrameHostTester::SimulateNavigationStart in simulation of
browser-initiated navigations. It also makes
TestWebContents::NavigateAndCommit use NavigationSimulator.

BUG=728571

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: Ib295868db2a3aab5828a178aba7b92d77ed9b2ba
Reviewed-on: https://chromium-review.googlesource.com/583096
Commit-Queue: Camille Lamy <clamy@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Timothy Loh <timloh@chromium.org>
Reviewed-by: Nathan Parker <nparker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#493442}
8 files changed