[snapshot] Track deserialization time in Memory benchmark

test-serialize/SerializationMemoryStats does not actually create a new
Isolate from scratch. Instead, it deserializes from the snapshot and
we can simply piggy-back off existing output to measure
deserialization time.

Bug: v8:6666,v8:7693
Change-Id: I8f709ea834ff7f5e46f7ebfa9b0c35d96095bf26
Reviewed-on: https://chromium-review.googlesource.com/1039585
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52918}

test/memory/Memory.json

@@ -25,6 +25,14 @@
     {
       "name": "SnapshotSizeContext",
       "results_regexp": "(\\d+) bytes for context #0$"
+    },
+    {
+      "name": "DeserializationTimeIsolate",
+      "results_regexp": "\\[Deserializing isolate \\(\\d+ bytes\\) took ([\\d.]+) ms\\]"
+    },
+    {
+      "name": "DeserializationTimeContext",
+      "results_regexp": "\\[Deserializing context #0 \\(\\d+ bytes\\) took ([\\d.]+) ms\\]"
     }
   ]
 }