Correctly name memory stat for context snapshot size.

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$"
     },
     {
       "name": "ReservedMemoryContext",
       "results_regexp": "(\\d+) bytes per context$"
     },
     {
       "name": "SnapshotSizeStartup",
       "results_regexp": "(\\d+) bytes for startup$"
     },
     {
-      "name": "SnapshotSizeStartup",
+      "name": "SnapshotSizeContext",
       "results_regexp": "(\\d+) bytes for context$"
     }
   ]
 }