json-parser does the same thing. This drastically reduces GC cost when
deserializing objects large enough that multiple collections are likely
(since it saves the effort to move them to the old generation, when we
know that the objects we create will persist at least until the deserializer
finishes).

The threshold was chosen to match json-parser.

This patch reduces deserialization time on a large blob of JSON-y data
by approximately 30%.

BUG=chromium:148757

Review-Url: https://codereview.chromium.org/2336973004
Cr-Commit-Position: refs/heads/master@{#39396}