As discussed in https://github.com/nodejs/llnode/issues/156, we need
postmortem metadata for Symbols to properly print Symbol property names
in postmortem debugging tools. Patch suggested by Ben Noordhuis
(https://github.com/nodejs/llnode/issues/156#issuecomment-350467852).

R=bmeurer@google.com, yangguo@google.com

Change-Id: Ied6d3c079e8b23a9c796bc632c37785ed7dbc118
Reviewed-on: https://chromium-review.googlesource.com/1205052Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55632}