src: associate is_main_thread() with worker_context()

In our codebase, the assumption generally is that `!is_main_thread()`
means that the current Environment belongs to a Node.js Worker thread.

Backport-PR-URL: #35241
PR-URL: #30467
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

Author: addaleax

src/api/environment.cc

@@ -332,8 +332,7 @@ Environment* CreateEnvironment(IsolateData* isolate_data,
       context,
       args,
       exec_args,
-      static_cast<Environment::Flags>(Environment::kIsMainThread |
-                                      Environment::kOwnsProcessState |
+      static_cast<Environment::Flags>(Environment::kOwnsProcessState |
                                       Environment::kOwnsInspector));
   env->InitializeLibuv(per_process::v8_is_profiling);
   if (env->RunBootstrapping().IsEmpty()) {

src/env-inl.h

@@ -854,7 +854,7 @@ inline void Environment::set_has_serialized_options(bool value) {
 }
 
 inline bool Environment::is_main_thread() const {
-  return flags_ & kIsMainThread;
+  return worker_context() == nullptr;
 }
 
 inline bool Environment::owns_process_state() const {

src/env.h

@@ -876,7 +876,6 @@ class Environment : public MemoryRetainer {
 
   enum Flags {
     kNoFlags = 0,
-    kIsMainThread = 1 << 0,
     kOwnsProcessState = 1 << 1,
     kOwnsInspector = 1 << 2,
   };

src/node_main_instance.cc

@@ -204,8 +204,7 @@ NodeMainInstance::CreateMainEnvironment(int* exit_code) {
       context,
       args_,
       exec_args_,
-      static_cast<Environment::Flags>(Environment::kIsMainThread |
-                                      Environment::kOwnsProcessState |
+      static_cast<Environment::Flags>(Environment::kOwnsProcessState |
                                       Environment::kOwnsInspector)) };
   env->InitializeLibuv(per_process::v8_is_profiling);
   env->InitializeDiagnostics();