UserspaceEmulator: Tidy up the MallocRegionMetadata construction a bit

Use designated initializers to make it very obvious what's being set,
and also give the metadata a backpointer to the region for convenience.

Userland/DevTools/UserspaceEmulator/MallocTracer.h

@@ -56,6 +56,7 @@ struct Mallocation {
 
 class MallocRegionMetadata {
 public:
+    MmapRegion& region;
     FlatPtr address { 0 };
     size_t chunk_size { 0 };