The prefix size is read from the Derived class.
The hash code of the object will be stored in this prefix.

Change-Id: I15646701c9bab99b8aed39ea9ad8ae03f6d605ed
Bug: v8:6443, v8:7569
Reviewed-on: https://chromium-review.googlesource.com/c/1337586
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58058}