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/1039585Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52918}