Add an initializer to the Serializer::allocation_size_ field so that the
array is value-initialized to zero.

Change-Id: I3bd9ecae63d9083e41cce4fbbfedc36ff5f0f741
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2502868
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70800}