[v8.x backport] inspector: Track async stack only when necessary

[ofrobots]

This is the 8.x backport of #16308 as the version of V8 in 8.x is slightly older than master and some changes were necessary.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

deps:v8, inspector

R= @nodejs/v8 @nodejs/v8-inspector

CI: https://ci.nodejs.org/job/node-test-pull-request/11104/
V8-CI: https://ci.nodejs.org/view/All/job/node-test-commit-v8-linux/1025/

[gibfahn]

@ofrobots looks like this conflicts with something I just landed, would you mind taking a look?

diff:

<<<<<<< HEAD
  // Enable tracking of async stack traces
  if (!enable_async_hook_function_.IsEmpty()) {
    Local<Function> enable_fn = enable_async_hook_function_.Get(isolate);
    auto result = enable_fn->Call(context, Undefined(isolate), 0, nullptr);
    if (result.IsEmpty()) {
      FatalError(
        "node::InspectorAgent::StartIoThread",
        "Cannot enable Inspector's AsyncHook, please report this.");
    }
  }

||||||| parent of 2069010344... inspector: track async stacks when necessary
  // Enable tracking of async stack traces
  if (!enable_async_hook_function_.IsEmpty()) {
    Local<Function> enable_fn = enable_async_hook_function_.Get(isolate);
    auto context = parent_env_->context();
    auto result = enable_fn->Call(context, Undefined(isolate), 0, nullptr);
    if (result.IsEmpty()) {
      FatalError(
        "node::InspectorAgent::StartIoThread",
        "Cannot enable Inspector's AsyncHook, please report this.");
    }
  }

=======
>>>>>>> 2069010344... inspector: track async stacks when necessary

[ofrobots]

@gibfahn Conflicts resolved.

[MylesBorins]

landed in ab0d7a6...2ea25ad