Add supports for nested breakdown in 'dmprof cat'.

tools/deep_memory_profiler/sorter.vm-sharing.json

 {
   "type": "sorter",
   "version": 1,
   "world": "vm",
   "name": "sharing",
+  "root": true,
   "order": {},
   "rules": [
     {
       "name": "others",
       "sharedwith": ["others"]
     },
     {
       "name": "chrome",
       "sharedwith": ["group"]
     },
     {
       "name": "private",
       "sharedwith": ["private"]
     },
     {
       "name": "any"
     }
   ]
 }