This will allow the devtools UI to display private fields on the scope
panel.

Instead of extending GetInternalProperties, we expose a separate
GetPrivateFields method on the debug interface. This allows us to do
better type checking, for example, we can directly cast to a
v8::Private as this can only contain private fields.

This also allows us to have better constraints on the input type --
v8::Object, as opposed to a v8::Value.

The KeyAccumulator is extended to collect private names for the
PRIVATE_NAMES_ONLY PropertyFilter.

Bug: v8:8773
Change-Id: Id47c551186c59dae9a06721074ef78144f25892f
Reviewed-on: https://chromium-review.googlesource.com/c/1475664
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59920}