Use the LibWebView history mirror to preserve traversable session
history across WebContent process swaps. WebContent reports snapshots to
the UI process, and new renderers can be seeded from the mirror.
Browser back and forward now resolve through the UI-owned used history
steps. WebContent still runs the spec traversal path when the current
renderer has enough matching state to do so.
Handle canceled and no-op UI navigations without leaving speculative
history entries or pending WebDriver waits behind. Preserve traversal
precheck state across synchronous IPC shutdown, and avoid overwriting a
restored target entry's persisted scroll state before the document has
adopted that entry.
Instead of wrapping all non-movable members of TransportSocket in OwnPtr
to keep it movable, make TransportSocket itself non-movable and wrap it
in OwnPtr.
This has been a longstanding ergonomic issue with our IPC compiler. Non-
trivial types were previously passed by const&. So if we wanted to avoid
expensive copies, we would have to const_cast and move the data.
We now pass ownership of all transferred data to the client subclasses.
This allows us to remove const_cast from these methods, and allows us to
avoid some trivial expensive copies that we didn't bother to const_cast.
