[vm] Sync ffi callbacks can enter owned target isolate

Adds a new code path for NativeCallable.isolateLocal invocations. If
the current thread is not entered into any isolate, but owns the
target isolate, then it enters the target isolate, invokes, then exits
the isolate.

Fixes: https://github.com/dart-lang/sdk/issues/61623
TEST=tests/ffi/function_callbacks_isolate_ownership_test.dart
Change-Id: I401f185fadf7d2a55190dafd15387e1c418c67c9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/452380
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Liam Appelbe <liama@google.com>

runtime/vm/dart_api_impl.cc

@@ -2232,6 +2232,16 @@ DART_EXPORT void Dart_SetCurrentThreadOwnsIsolate() {
   }
 }
 
+DART_EXPORT void Dart_ClearCurrentThreadOwnsIsolate_ForTesting() {
+  Isolate* isolate = Isolate::Current();
+  CHECK_ISOLATE(isolate);
+  if (!isolate->SetOwnerThread(OSThread::GetCurrentThreadId(),
+                               OSThread::kInvalidThreadId)) {
+    FATAL("Tried to clear ownership of isolate %s, but we don't own it\n",
+          isolate->name());
+  }
+}
+
 DART_EXPORT bool Dart_GetCurrentThreadOwnsIsolate(Dart_Port port) {
   return PortMap::IsOwnedByCurrentThread(port);
 }