Stage @@toStringTag (--harmony-tostring)
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R=rossberg@chromium.org,caitpotter88@gmail.com
LOG=Y

Review URL: https://codereview.chromium.org/768833002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#25573}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh