The WebRequest API needs to know if a given window ID is at the top level, for
various reasons. It currently figures this out by mapping a channel's load
context to a <browser> element, which tracks its current top outer window ID.
But this is inefficient, and not friendly to C++ callers.
Adding the top window ID to the load info simplifies things considerably.
MozReview-Commit-ID: Fy0gxTqQZMZ
--HG--
extra : rebase_source : bb5b1e1b3294004ca5e713fc88c4e20652296e53