Embedder may implement any non trivial logic using embedder fields,
e.g. creates wrappers lazily and store native pointer to native object
inside embedder fields.

R=yangguo@chromium.org

Bug: v8:7588
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Id14eb6e686f8945cb350cfe77e8a4b3c52c5ec5b
Reviewed-on: https://chromium-review.googlesource.com/1013404Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52741}