Cherry-Pick: Delay setting initial memory block until it officially exists.

BUG=323017
R=robertphillips@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/99833002

Not submitting a separate patch to QtWebEngine.
This change is known to be contained in our next snapshot.

Change-Id: Id267513b61ebc89446808206755661ae946e0a45
Reviewed-by: Andras Becsi <andras.becsi@digia.com>