PlzNavigate: properly stop navigations in WebContents::Stop

This CL rewrites WebContentsImpl::Stop so that it stops all loads even in
browser-side-navigation mode. In particular, the speculative RenderFrameHost
should be told to stop, and any NavigationRequest in the FrameTree should be
destroyed.

BUG=504347

Review URL: https://codereview.chromium.org/1226773008

Cr-Commit-Position: refs/heads/master@{#337801}
4 files changed