src: add public API to expose the main V8 Platform

Add an API to get MultiIsolatePlatform used in node main thread.

PR-URL: #20447
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node.cc

@@ -4439,6 +4439,11 @@ void FreeEnvironment(Environment* env) {
 }
 
 
+MultiIsolatePlatform* GetMainThreadMultiIsolatePlatform() {
+  return v8_platform.Platform();
+}
+
+
 MultiIsolatePlatform* CreatePlatform(
     int thread_pool_size,
     v8::TracingController* tracing_controller) {

src/node.h

@@ -251,6 +251,11 @@ NODE_EXTERN Environment* CreateEnvironment(IsolateData* isolate_data,
 NODE_EXTERN void LoadEnvironment(Environment* env);
 NODE_EXTERN void FreeEnvironment(Environment* env);
 
+// This returns the MultiIsolatePlatform used in the main thread of Node.js.
+// If NODE_USE_V8_PLATFORM haven't been defined when Node.js was built,
+// it returns nullptr.
+NODE_EXTERN MultiIsolatePlatform* GetMainThreadMultiIsolatePlatform();
+
 NODE_EXTERN MultiIsolatePlatform* CreatePlatform(
     int thread_pool_size,
     v8::TracingController* tracing_controller);