Shell::SerializeValue was using a HandleScope, but was also storing Handles in
an ObjectList. The ObjectList handles would persist after the function had
returned, but will have already been destroyed by the HandleScope, so there is
a use-after-free.

This change removes the HandleScope in Shell::SerializeValue and relies on the
caller's HandleScope.

BUG=chromium:503968
R=jochen@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1211433003

Cr-Commit-Position: refs/heads/master@{#29265}