In order for same-origin NavigableContainers (iframe, frame, embed, ...)
and window.open() WindowProxies to have the proper JS access to their
embedder/opener, we need to host multiple top level traversables in the
same WebContent process. As a first step, make WebContent::PageHost hold
a HashMap of PageClient objects, each holding their own Web::Page that
represents a TraversableNavigable's API surface with the UI process.
