The number of embedder fields grows dynamically, but reading these
fields do not perform bounds checks. The naming is taken from a similar
method on v8::Isolate.

Also changed the growing strategy for the backing store to not
over-allocate.

R=adamk@chromium.org, bmeurer@chromium.org

Bug: v8:7533
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I70beab124a32296c940ffabd897a7790bc8ff47a
Reviewed-on: https://chromium-review.googlesource.com/952923Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51822}