chore: DCHECK that events are only emitted on the UI thread (#15873)

* chore: DCHECK that events are only emitted on the UI thread

* move dcheck to EmitWithEvent

atom/browser/api/event_emitter.h

@@ -8,6 +8,7 @@
 #include <vector>
 
 #include "atom/common/api/event_emitter_caller.h"
+#include "content/public/browser/browser_thread.h"
 #include "native_mate/wrappable.h"
 
 namespace content {
@@ -97,6 +98,7 @@ class EventEmitter : public Wrappable<T> {
   bool EmitWithEvent(const base::StringPiece& name,
                      v8::Local<v8::Object> event,
                      const Args&... args) {
+    DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
     v8::Locker locker(isolate());
     v8::HandleScope handle_scope(isolate());
     EmitEvent(isolate(), GetWrapper(), name, event, args...);