Fix focus to commit message

After clicking edit button, editable-content tried to put focus into
textarea. However iron-autogrow-textarea was not yet rendered so
it throw js error:
Error: selector 'iron-autogrow-textarea' did not match anything'

Google-Bug-Id: b/228800894
Release-Notes: skip
Change-Id: I51a48f84f750798d6a93a00ca765b308341b7517
diff --git a/polygerrit-ui/app/elements/shared/gr-editable-content/gr-editable-content.ts b/polygerrit-ui/app/elements/shared/gr-editable-content/gr-editable-content.ts
index 99b72fa..bdd2b52 100644
--- a/polygerrit-ui/app/elements/shared/gr-editable-content/gr-editable-content.ts
+++ b/polygerrit-ui/app/elements/shared/gr-editable-content/gr-editable-content.ts
@@ -424,8 +424,9 @@
     }
   }
 
-  handleEditCommitMessage() {
+  async handleEditCommitMessage() {
     this.editing = true;
+    await this.updateComplete;
     this.focusTextarea();
   }
 }