src: keep track of env properly in node_perf.cc

[addaleax]

First commit is to avoid conflicts with the second, the second commit is needed for the “real” one.

---

Currently, measuring GC timing using node_perf is somewhat broken, because Isolates and Node Environments do not necessarily match 1:1; each environment adds its own hook, so possibly the hook code runs multiple times, but since it can’t reliably compute its corresponding event loop based on the Isolate, each run targets the same Environment right now.

This fixes that problem by using new overloads of the GC tracking APIs that can pass data to the callback through opaque pointers.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

deps/v8, src/node_perf

@nodejs/v8 @jasnell