When the embedder sets the heap tracer, V8, during marking, will collect all reachable wrappers, and then ask embedder to trace its heap. The embedder is expected to call PersistentBase::RegisterExternalReference with all wrappers reachable from the given ones. This fixed point iteration happens in MarkCompact::ProcessEphemeralMarking.

For more efficient object visiting during marking, we need a special JS_API_OBJECT_TYPE (in tandem with already existing JS_SPECIAL_API_OBJECT_TYPE) and corresponding visitor (JSApiObjectVisitor).

BUG=chromium:468240
LOG=no

Review URL: https://codereview.chromium.org/1844413002

Cr-Commit-Position: refs/heads/master@{#35412}