This patch adds an IPC call for debugging requests. It's stringly typed
and very simple, and allows us to easily implement all the features in
the Browser's Debug menu.
To support this, the GUI process and the WebContent service will now
coordinate their backing store bitmaps. Each backing store can be
referred to by a serial ID, and we don't need to keep resending it
as a file descriptor.
We should probably do something similar in WindowServer. :^)
