Creates the nsDocShellLoadState object, which is basically
nsDocShellLoadInfo plus a few extra fields to make it usable as a
single argument to nsDocShell::LoadURI (and eventually
nsDocShell::InternalLoad).

Subframe history handling is a huge logic block in
nsDocShell::LoadURI, which is only used on history loads. This patch
also extracts the logic out into its own function to make the body of
LoadURI clearer.

Differential Revision: https://phabricator.services.mozilla.com/D6944

--HG--
rename : docshell/base/nsDocShellLoadInfo.cpp => docshell/base/nsDocShellLoadState.cpp
rename : docshell/base/nsDocShellLoadInfo.h => docshell/base/nsDocShellLoadState.h
extra : moz-landing-system : lando