The members of HandleScopeImplementer are copied with memcpy when
the isolate is transferred to another thread. List contained some
primitives which allowed us to manually free the backing store, which
was needed in order to ensure that threads would not hold on to
old pointers and use them later. With std::vector, we can't do that.

Here we change the HandleScopeImplementer to instead use a custom
structure DetachableVector, which contains a std::vector but allows
manual detaching and freeing of the backing store. This allows us to
maintain the old behavior.

Bug: v8:6333
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I6361d161cdb19878ba19ed51d6ba2fae99e8cdc0
Reviewed-on: https://chromium-review.googlesource.com/660125Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48197}