Remove ScriptValueSerializer::serializeNullValue.

Replace it with a hard-coded encoding of "null" (at version 9, though it
doesn't much matter since we are be backward-compatible).

BUG=690132

Review-Url: https://codereview.chromium.org/2684593006
Cr-Commit-Position: refs/heads/master@{#449173}
4 files changed