src: stop leaking a CppHeap in CommonEnvironmentSetup

`CommonEnvironmentSetup` created a `CppHeap` for its `CreateParams`
before deciding how to create the isolate, but `NewIsolate()` ignores
`params->cpp_heap` and attaches a heap of its own (or the one from
`IsolateSettings`). The first heap was never attached or destroyed, so
every non-snapshotting setup leaked one `CppHeap`; cppgc's heap
registry keeps it reachable, which is why LSAN stays quiet about it.

Only create the heap on the snapshotting path, where the params go to
the `SnapshotCreator` directly.

Refs: #55337
Signed-off-by: Shelley Vohr <shelley.vohr@gmail.com>
PR-URL: #65792
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Jake Yuesong Li <jake.yuesong@gmail.com>

Author: codebytere

src/api/embed_helpers.cc

@@ -117,8 +117,6 @@ CommonEnvironmentSetup::CommonEnvironmentSetup(
   Isolate::CreateParams params;
   params.array_buffer_allocator = impl_->allocator.get();
   params.external_references = external_references.data();
-  params.cpp_heap =
-      v8::CppHeap::Create(platform, v8::CppHeapCreateParams{{}}).release();
 
   Isolate* isolate;
 
@@ -130,6 +128,8 @@ CommonEnvironmentSetup::CommonEnvironmentSetup(
     // isolate, so that the memory reducer can be initialized.
     isolate = impl_->isolate = Isolate::Allocate(GetOrCreateIsolateGroup());
     platform->RegisterIsolate(isolate, loop);
+    params.cpp_heap =
+        v8::CppHeap::Create(platform, v8::CppHeapCreateParams{{}}).release();
 
     impl_->snapshot_creator.emplace(isolate, params);
     isolate->SetCaptureStackTraceForUncaughtExceptions(