Another pure refactoring CL. This extracts methods used only by Deserializer
subclasses.

Bug: v8:6624
Change-Id: Ib4dd7cdc591dff217e282e68a490c8c7129b9c96
Reviewed-on: https://chromium-review.googlesource.com/602188
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47211}