Fix Hangul typing does not work properly

compositionstart event may fire in the meantime because
compositionend event is being processed asynchronously.

Signed-off-by: JungEon Kim <me@jungeon.kim>

src/components/views/rooms/BasicMessageComposer.js

@@ -203,9 +203,9 @@ export default class BasicMessageEditor extends React.Component {
         if (isSafari) {
             this._onInput({inputType: "insertCompositionText"});
         } else {
-            setTimeout(() => {
+            Promise.resolve().then(() => {
                 this._onInput({inputType: "insertCompositionText"});
-            }, 0);
+            });
         }
     }