[snapshot] support multiple contexts in the same snapshot.

test/memory/Memory.json

 {
   "name": "Memory",
   "run_count": 5,
   "units": "bytes",
   "path" : ["."],
   "binary": "cctest",
   "main": "test-serialize/SerializationMemoryStats",
   "tests": [
     {
       "name": "ReservedMemoryIsolate",
       "results_regexp": "(\\d+) bytes per isolate$"
     },
     {
       "name": "ReservedMemoryContext",
-      "results_regexp": "(\\d+) bytes per context$"
+      "results_regexp": "(\\d+) bytes per context #0$"
     },
     {
       "name": "SnapshotSizeStartup",
       "results_regexp": "(\\d+) bytes for startup$"
     },
     {
       "name": "SnapshotSizeContext",
-      "results_regexp": "(\\d+) bytes for context$"
+      "results_regexp": "(\\d+) bytes for context #0$"
     }
   ]
 }