Landing changes to wrap XUL windows in a box frame so that we can support

intrinsic sizing of windows.  (Unlike HTML and XML, we are no longer wrapped
in an area frame. We are instead wrapped in a box frame.)

Also included is a patch for the table code, since it crashes inside boxes
because of a bug in its incremental reflow code.

nsISpaceManager was touched to give it a GetIID method so that it could be
used with nsCOMPtrs.

layout/tables/nsTableOuterFrame.cpp

@@ -320,6 +320,9 @@ nsresult nsTableOuterFrame::IR_TargetIsChild(nsIPresContext&        aPresContext
 {
   TDBG_S(gsDebugIR,"TOF IR: IR_TargetIsChild\n");
   nsresult rv;
+  if (!aNextFrame)
+    return NS_OK;
+
   if (aNextFrame == mInnerTableFrame) {
     rv = IR_TargetIsInnerTableFrame(aPresContext, aDesiredSize, aReflowState, aStatus);
   }